title: --proto_path passed empty directory name
body: <p>I'm trying to compile a proto file with the following command:</p>
  <pre><code>protoc -I=. --python_out=. ./message.proto --proto_path=.
  </code></pre>
  <p>But I'm getting this error:</p>
  <pre><code>--proto_path passed empty directory name.  (Use &quot;.&quot; for current directory.)
  </code></pre>
  <p>What to do?</p>