Set Default Value for ServerUrl.Domain

- Allow users to skip setting the value for ServerUrl.Domain in the
proxy.config file if it's equal to empty string.
- Otherwise, proxy fails if user doesn't set a value for domain.
- Related to #118
- This will allow this configuration
Sample configuration: <serverUrl
url="http://artemisf/arcgis/rest/services/California/MapServer"
matchAll="true" username="XXX" password="XXX"/>