title: _Nullable method in Objective-C
body: <p>If I have a method:</p>
  
  <pre><code>- (NSString*)convertName;
  </code></pre>
  
  <p>And then I do something like:</p>
  
  <pre><code>- (NSString*)convertName {
      if (![myName isEqualToString:@"someString"]) {
          return NULL;
      }
      .......
  }
  </code></pre>
  
  <p>Why the compiler is letting me do this if I didn't specify _Nullable?</p>