extract method: allow to extract @NotNull code with missed branches which would be replaced with 'return null' with check on call site (IDEA-120076)

java/java-tests/testData/refactoring/extractMethod/ExitPoints8_after.java

@@ -0,0 +1,16 @@
+class C {
+    public Object m() {
+        Object o = newMethod();
+        if (o != null) return o;
+        return null;
+    }
+
+    private Object newMethod() {
+        for (Object o : new ArrayList<Object>()) {
+            if (o != null) {
+                return o;
+            }
+        }
+        return null;
+    }
+}