test: improve coverage for Strings utility class

Added missing tests for empty edge cases in countOccurrences and added coverage for the overloaded join methods utilizing StringBuilder.

Co-authored-by: RoiSoleil <3462260+RoiSoleil@users.noreply.github.com>

Author: google-labs-jules[bot]

org.moreunit.core.test/test/org/moreunit/core/util/StringsTest.java

@@ -83,6 +83,16 @@ public void should_count_occurrences() throws Exception
         assertThat(Strings.countOccurrences("bla<>h<>", "<>")).isEqualTo(2);
     }
 
+    @Test
+    public void should_return_zero_when_count_occurrences_is_called_with_empty_strings() throws Exception
+    {
+        assertThat(Strings.countOccurrences("", "test")).isEqualTo(0);
+        assertThat(Strings.countOccurrences(null, "test")).isEqualTo(0);
+        assertThat(Strings.countOccurrences("test", "")).isEqualTo(0);
+        assertThat(Strings.countOccurrences("test", null)).isEqualTo(0);
+        assertThat(Strings.countOccurrences(null, null)).isEqualTo(0);
+    }
+
     @Test
     public void split_should_ignore_blank_parts() throws Exception
     {
@@ -128,6 +138,22 @@ public void join_should_join_given_strings_whith_given_separator() throws Except
         assertThat(Strings.join(" -> ", "aBc", "2", "DEf")).isEqualTo("aBc -> 2 -> DEf");
     }
 
+    @Test
+    public void join_should_append_to_string_builder_with_given_separator_for_collection() throws Exception
+    {
+        StringBuilder sb = new StringBuilder("prefix: ");
+        Strings.join(sb, ",", java.util.Arrays.asList("1", "2", "3"));
+        assertThat(sb.toString()).isEqualTo("prefix: 1,2,3");
+    }
+
+    @Test
+    public void join_should_append_to_string_builder_with_given_separator_for_array() throws Exception
+    {
+        StringBuilder sb = new StringBuilder("prefix: ");
+        Strings.join(sb, " -> ", new String[]{"aBc", "2", "DEf"});
+        assertThat(sb.toString()).isEqualTo("prefix: aBc -> 2 -> DEf");
+    }
+
     @Test
     public void emptyArray_should_return_empty_array()
     {