Fixed XML parse error when loading raw metadata with 'inline' parameter.

An error occurs in MetaData.load() because the value of self.metadata is empty string.
By applying the patch it works well.

Or I wonder if my configuration is not right.
The following is part of dict for sp_config.
```python

    'metadata': {
        'inline': ['<?xml version="1.0"?>......'],
    },
```