Avoid CompressionError for newlines in stderr

UglifyJS2 prints two newlines '\n\n' into stderr for me.
The `if stderr` in the `SubProcessCompressor.execute_command` raises a CompressionError if that happens.
I've added a `strip()` call to avoid that.

pipeline/compressors/__init__.py

@@ -232,7 +232,7 @@ class SubProcessCompressor(CompressorBase):
         if content:
             content = smart_bytes(content)
         stdout, stderr = pipe.communicate(content)
-        if stderr:
+        if stderr.strip():
             raise CompressorError(stderr)
         if self.verbose:
             print(stderr)