Fix indent for empty string, add tests

Author: zbynek

user/super/com/google/gwt/emul/java/lang/String.java

@@ -815,11 +815,13 @@ private int getTrailingWhitespaceLength() {
   }
 
   public String indent(int spaces) {
-    if (spaces  == 0) {
-      return this;
+    if (isEmpty()) {
+      return "";
     }
     Stream<String> indentedLines;
-    if (spaces > 0) {
+    if (spaces == 0) {
+      indentedLines = lines();
+    } else if (spaces > 0) {
       String spaceString = " ".repeat(spaces);
       indentedLines = lines().map(line -> spaceString + line);
     } else {

user/test-super/com/google/gwt/emultest/super/com/google/gwt/emultest/java17/lang/StringTest.java

@@ -28,8 +28,23 @@ public void testTransform() {
   }
 
   public void testIndent() {
+    assertEquals("", hideFromCompiler("").indent(0));
+    assertEquals("", hideFromCompiler("").indent(2));
+    assertEquals("", hideFromCompiler("").indent(-2));
+    assertEquals(" \n", hideFromCompiler(" ").indent(0));
+    assertEquals("   \n", hideFromCompiler(" ").indent(2));
+    assertEquals("\n", hideFromCompiler(" ").indent(-2));
+    assertEquals("x\n", hideFromCompiler("x").indent(0));
     assertEquals("  x\n", hideFromCompiler("x").indent(2));
+    assertEquals("x\n", hideFromCompiler("x").indent(-2));
+    assertEquals(" x \n", hideFromCompiler(" x ").indent(0));
+    assertEquals("   x \n", hideFromCompiler(" x ").indent(2));
+    assertEquals("x \n", hideFromCompiler(" x ").indent(-2));
     assertEquals("x\n", hideFromCompiler("  x").indent(-2));
+    assertEquals(" x \n", hideFromCompiler(" \t x ").indent(-2));
+    assertEquals("x\ny\n", hideFromCompiler("x\ny").indent(0));
+    assertEquals("x\ny\n", hideFromCompiler("x\r\ny").indent(0));
+    assertEquals("x\ny\n", hideFromCompiler("x\ry").indent(0));
     assertEquals("  x\n  y\n", hideFromCompiler("x\ny").indent(2));
     assertEquals("  x\n  y\n", hideFromCompiler("x\r\ny").indent(2));
     assertEquals("  x\n  y\n", hideFromCompiler("x\ry").indent(2));