Fix YcmShowDetailedDiagnostic command on line without a diagnostic

The YcmShowDetailedDiagnostic command raises a NoneType exception on a line
with no diagnostic.

doc/youcompleteme.txt

@@ -1900,8 +1900,8 @@ The *GoToDeclaration* subcommand
 
 Looks up the symbol under the cursor and jumps to its declaration.
 
-Supported in filetypes: 'c, cpp, objc, objcpp, cuda, cs, go, java, python, rust,
-typescript'
+Supported in filetypes: 'c, cpp, objc, objcpp, cuda, cs, go, java, python,
+rust, typescript'
 
 -------------------------------------------------------------------------------
 The *GoToDefinition* subcommand

python/ycm/tests/youcompleteme_test.py

@@ -386,6 +386,22 @@ def YouCompleteMe_ShowDetailedDiagnostic_MessageFromServer_test(
   ] )
 
 
+@YouCompleteMeInstance()
+@patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
+def YouCompleteMe_ShowDetailedDiagnostic_Exception_test(
+  ycm, post_vim_message ):
+
+  current_buffer = VimBuffer( 'buffer' )
+  with MockVimBuffers( [ current_buffer ], current_buffer ):
+    with patch( 'ycm.client.base_request._JsonFromFuture',
+                side_effect = RuntimeError( 'Some exception' ) ):
+      ycm.ShowDetailedDiagnostic(),
+
+  post_vim_message.assert_has_exact_calls( [
+    call( 'Some exception', truncate = False )
+  ] )
+
+
 @YouCompleteMeInstance()
 @patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
 def YouCompleteMe_ShowDiagnostics_FiletypeNotSupported_test( ycm,

python/ycm/youcompleteme.py

@@ -660,7 +660,7 @@ class YouCompleteMe( object ):
     detailed_diagnostic = BaseRequest().PostDataToHandler(
         BuildRequestData(), 'detailed_diagnostic' )
 
-    if 'message' in detailed_diagnostic:
+    if detailed_diagnostic and 'message' in detailed_diagnostic:
       vimsupport.PostVimMessage( detailed_diagnostic[ 'message' ],
                                  warning = False )