ChangeToAppendUtil: do not add useless .append("")

Partially fixes IDEA-180178 String concatenation as argument to 'StringBuilder.append()' call should be smarter

java/java-analysis-impl/src/com/intellij/codeInspection/util/ChangeToAppendUtil.java

@@ -56,7 +56,7 @@ public class ChangeToAppendUtil {
           }
           builder.append(CommentTracker.textWithSurroundingComments(operand));
         }
-        else {
+        else if (!operand.textMatches("\"\"")) {
           isConstant = false;
           if (builder.length() != 0) {
             append(builder, useStringValueOf && !isString, out);

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/stringConcatInLoop/afterConcatWithEmpty.java

@@ -0,0 +1,11 @@
+// "Convert variable 'res' from String to StringBuilder" "true"
+
+public class Main {
+    String test(int[] ints) {
+        StringBuilder res = new StringBuilder();
+        for (int i : ints) {
+            res.append(i);
+        }
+        return res.toString();
+    }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/stringConcatInLoop/beforeConcatWithEmpty.java

@@ -0,0 +1,11 @@
+// "Convert variable 'res' from String to StringBuilder" "true"
+
+public class Main {
+    String test(int[] ints) {
+        String res = "";
+        for (int i : ints) {
+            res = res <caret>+ i + "";
+        }
+        return res;
+    }
+}