Bailout for duplicateBranch check if branches are empty (#5354)

lib/checkother.cpp

@@ -2021,7 +2021,7 @@ void CheckOther::checkDuplicateBranch()
             std::string branch2 = scope->classEnd->tokAt(3)->stringifyList(scope->classEnd->linkAt(2));
 
             // check for duplicates
-            if (branch1 == branch2)
+            if (!branch1.empty() && branch1 == branch2)
                 duplicateBranchError(scope->classDef, scope->classEnd->next());
         }
     }

test/testother.cpp

@@ -3759,6 +3759,16 @@ private:
               "    frac = front/((front-back));\n"
               "}");
         ASSERT_EQUALS("[test.cpp:5] -> [test.cpp:3]: (style, inconclusive) Found duplicate branches for 'if' and 'else'.\n", errout.str());
+
+        // No message about empty branches (#5354)
+        check("void f()\n"
+              "{\n"
+              "  if (front < 0)\n"
+              "  {}\n"
+              "  else\n"
+              "  {}\n"
+              "}");
+        ASSERT_EQUALS("", errout.str());
     }
 
     void duplicateBranch2() {