Directly return empty string when not using existing runner

- This should have been happening anyway due to a later conditional,
      but may not if tmux changed its behaviour.

plugin/vimux.vim

@@ -235,7 +235,9 @@ function! s:existingRunnerId() abort
   let runnerType = VimuxOption('VimuxRunnerType')
   let query = get(VimuxOption('VimuxRunnerQuery'), runnerType, '')
   if empty(query)
-    if !empty(VimuxOption('VimuxUseNearest'))
+    if empty(VimuxOption('VimuxUseNearest'))
+      return ''
+    else
       return s:nearestRunnerId()
     endif
   endif