Fix a StringIndexOutOfBoundsException in MissingSummary

Discovered while validating 2.4.0 release (#1639)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=313480851

Author: cushon

core/src/main/java/com/google/errorprone/bugpatterns/javadoc/MissingSummary.java

@@ -106,7 +106,10 @@ private Description handle(@Nullable DocTreePath docTreePath, VisitorState state
     }
     ReturnTree returnTree = findFirst(docTreePath, ReturnTree.class);
     if (returnTree != null) {
-      return generateReturnFix(docTreePath, returnTree, state);
+      Description description = generateReturnFix(docTreePath, returnTree, state);
+      if (!description.equals(NO_MATCH)) {
+        return description;
+      }
     }
     SeeTree seeTree = findFirst(docTreePath, SeeTree.class);
     if (seeTree != null) {
@@ -127,6 +130,9 @@ private Description generateReturnFix(
       DocTreePath docTreePath, ReturnTree returnTree, VisitorState state) {
     int pos = ((DCDocComment) docTreePath.getDocComment()).comment.getSourcePos(0);
     String description = returnTree.toString().replaceAll("^@return ", "");
+    if (description.isEmpty()) {
+      return NO_MATCH;
+    }
     SuggestedFix fix =
         SuggestedFix.builder()
             .merge(Utils.replace(returnTree, "", state))

core/src/test/java/com/google/errorprone/bugpatterns/javadoc/MissingSummaryTest.java

@@ -209,4 +209,17 @@ public void seeWithHtmlLink() {
             "public interface Test {}")
         .doTest();
   }
+
+  @Test
+  public void emptyReturn() {
+    helper
+        .addSourceLines(
+            "Test.java", //
+            "interface Test {",
+            "  // BUG: Diagnostic contains:",
+            "  /** @return */",
+            "  int test(int n);",
+            "}")
+        .doTest();
+  }
 }