Prevent leading blank lines in list items when formatting block tags.

* Update `writeTableOpen`, `writePreOpen`, and `writeBlockquoteOpenOrClose` in `JavadocWriter` to check `if (wroteAnythingSignificant && !atStartOfLine)` before requesting a blank line. This matches existing logic in `writeMarkdownTable` (`line 378`) and `writeMarkdownFencedCodeBlock` (`line 346`), ensuring that block elements opening immediately at the start of a list item (`<li>` or `- `) do not get preceded by an unnecessary blank line.
* Fix a `NullPointerException` in `MarkdownPositions.visitListItem` when a Markdown list item has a non-`Paragraph` first child (such as `- <table>...</table>`), safely falling through when `firstChild` is not an instance of `Paragraph`.
* Update unit tests in `JavadocFormattingTest` (`tableInHtmlListItem` and `tableInMarkdownListItem`) to verify correct formatting and verify no `NullPointerException` is thrown when `- <table>` is lexed.

PiperOrigin-RevId: 945755137

Author: kluever

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocWriter.java

@@ -260,7 +260,7 @@ void writeParagraphOpen(Token token) {
   }
 
   void writeBlockquoteOpenOrClose(Token token) {
-    if (wroteAnythingSignificant) {
+    if (wroteAnythingSignificant && !atStartOfLine) {
       requestBlankLine();
     }
 
@@ -270,7 +270,7 @@ void writeBlockquoteOpenOrClose(Token token) {
   }
 
   void writePreOpen(PreOpenTag token) {
-    if (wroteAnythingSignificant) {
+    if (wroteAnythingSignificant && !atStartOfLine) {
       requestBlankLine();
     }
 
@@ -292,7 +292,7 @@ void writeCodeClose(CodeCloseTag token) {
   }
 
   void writeTableOpen(TableOpenTag token) {
-    if (wroteAnythingSignificant) {
+    if (wroteAnythingSignificant && !atStartOfLine) {
       requestBlankLine();
     }
 

core/src/main/java/com/google/googlejavaformat/java/javadoc/MarkdownPositions.java

@@ -129,17 +129,16 @@ private boolean visitListItem(ListItem listItem) {
       verify(matcher.lookingAt());
       ListItemOpenTag openToken = new ListItemOpenTag(matcher.group(1));
       addSpan(listItem, openToken, LIST_ITEM_CLOSE_TOKEN);
-      return switch (listItem.getFirstChild()) {
-        case Paragraph paragraph -> {
-          // A ListItem typically contains a Paragraph, but we don't want to visit that Paragraph
-          // because that would lead us to introduce a line break after the list introduction
-          // (the `-` or whatever). So we visit the children and siblings of the Paragraph instead.
-          visitNodeList(paragraph.getFirstChild());
-          visitNodeList(paragraph.getNext());
-          yield true;
-        }
-        default -> false;
-      };
+      Node firstChild = listItem.getFirstChild();
+      if (firstChild instanceof Paragraph paragraph) {
+        // A ListItem typically contains a Paragraph, but we don't want to visit that Paragraph
+        // because that would lead us to introduce a line break after the list introduction
+        // (the `-` or whatever). So we visit the children and siblings of the Paragraph instead.
+        visitNodeList(paragraph.getFirstChild());
+        visitNodeList(paragraph.getNext());
+        return true;
+      }
+      return false;
     }
 
     private void visitHeading(Heading heading) {

core/src/test/java/com/google/googlejavaformat/java/JavadocFormattingTest.java

@@ -2216,14 +2216,10 @@ public void tableInHtmlListItem() {
         /// </ul>
         class Test {}
         """;
-    // requestBlankLine() in writeTableOpen() inserts a blank line after <li> before <table> when
-    // inside a list item.
     String expected =
         """
         /// <ul>
-        ///   <li>
-        ///
-        ///       <table>
+        ///   <li><table>
         /// <tr><td>Foo</td></tr>
         /// </table>
         ///
@@ -2233,6 +2229,27 @@ class Test {}
     doFormatTest(input, expected);
   }
 
+  @Test
+  public void tableInMarkdownListItem() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// - <table>
+        ///   <tr><td>Foo</td></tr>
+        ///   </table>
+        class Test {}
+        """;
+    String expected =
+        """
+        ///
+        /// - <table>
+        ///   <tr><td>Foo</td></tr>
+        ///   </table>
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
   @Test
   public void tableInsideMarkdownListItemAfterText() {
     assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
@@ -2244,8 +2261,6 @@ public void tableInsideMarkdownListItemAfterText() {
         ///   </table>
         class Test {}
         """;
-    // requestBlankLine() in writeTableOpen() inserts a blank line after item text before <table>
-    // inside the list item.
     String expected =
         """
         ///