don't warn on empty regexp element

RegExpSupport/src/org/intellij/lang/regexp/inspection/DuplicateAlternationBranchInspection.java

@@ -56,6 +56,9 @@ public class DuplicateAlternationBranchInspection extends LocalInspectionTool {
       final RegExpBranch[] branches = pattern.getBranches();
       for (int i = 0; i < branches.length - 1; i++) {
         final RegExpBranch branch1 = branches[i];
+        if (branch1.getAtoms().length == 0) {
+          continue;
+        }
         for (int j = i + 1; j < branches.length; j++) {
           final RegExpBranch branch2 = branches[j];
           if (RegExpEquivalenceChecker.areElementsEquivalent(branch1, branch2)) {

RegExpSupport/test/org/intellij/lang/regexp/inspection/DuplicateAlternationBranchInspectionTest.java

@@ -35,6 +35,10 @@ public class DuplicateAlternationBranchInspectionTest extends RegExpInspectionTe
     highlightTest("<warning descr=\"Duplicate branch in alternation\">[abc]</warning>|<warning descr=\"Duplicate branch in alternation\">[cba]</warning>");
   }
 
+  public void testEmptyBranches() {
+    highlightTest("|||");
+  }
+
   @NotNull
   @Override
   protected LocalInspectionTool getInspection() {