OptionalToIfInspection: misc fixes

1. generate if with curly braces when it contains single declaration inside
2. restore else branch after generating code for flatMap
3. do not reassign source variable for nested source operations

GitOrigin-RevId: 061fbcc8d8dda586dcecb9a448752709a43370da

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/optionalToIf/afterFlatMap.java

@@ -50,6 +50,20 @@ class Test {
       }
   }
 
+  void nestedFlatMap(String var0) {
+      boolean b = false;
+      if (var0 != null) {
+          String var2 = var0.toLowerCase();
+          b = true;
+      }
+  }
+
+  String flatMapWithOrInside() {
+      Object o1 = null;
+      String empty = null;
+      throw new NoSuchElementException("No value present");
+  }
+
   <T> T id(T t) {
     return t;
   }