Replace CharSequence.isEmpty() with length check

CharSequence.isEmpty() is a default method added in Java 15,
but the client module targets Java 11. Replace all four call
sites with the equivalent length() == 0 check to avoid
NoSuchMethodError at runtime on Java 11.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mtopolnik

core/src/main/java/io/questdb/client/cutlass/qwp/client/QwpUdpSender.java

@@ -778,7 +778,7 @@ private void beginColumnWrite(QwpTableBuffer.ColumnBuffer column, CharSequence c
         int columnIndex = column.getIndex();
         ensureStagedColumnMarkCapacity(columnIndex + 1);
         if (stagedColumnMarks[columnIndex] == currentRowMark) {
-            if (columnName != null && columnName.isEmpty()) {
+            if (columnName != null && columnName.length() == 0) {
                 throw new LineSenderException("designated timestamp already set for current row");
             }
             throw new LineSenderException("column '" + columnName + "' already set for current row");

core/src/main/java/io/questdb/client/cutlass/qwp/client/QwpWebSocketSender.java

@@ -486,7 +486,7 @@ public Sender decimalColumn(CharSequence name, Decimal256 value) {
 
     @Override
     public Sender decimalColumn(CharSequence name, CharSequence value) {
-        if (value == null || value.isEmpty()) return this;
+        if (value == null || value.length() == 0) return this;
         checkNotClosed();
         checkTableSelected();
         try {
@@ -903,7 +903,7 @@ private void checkTableSelected() {
 
     private String checkedColumnName(CharSequence name) {
         if (name == null || !TableUtils.isValidColumnName(name, DEFAULT_MAX_NAME_LENGTH)) {
-            if (name == null || name.isEmpty()) {
+            if (name == null || name.length() == 0) {
                 throw new LineSenderException("column name cannot be empty");
             }
             if (name.length() > DEFAULT_MAX_NAME_LENGTH) {

core/src/main/java/io/questdb/client/cutlass/qwp/protocol/OffHeapAppendMemory.java

@@ -150,7 +150,7 @@ public void putShort(short value) {
      * Pre-ensures worst-case capacity to avoid per-byte checks.
      */
     public void putUtf8(CharSequence value) {
-        if (value == null || value.isEmpty()) {
+        if (value == null || value.length() == 0) {
             return;
         }
         int len = value.length();