Null Metadata Field Causing Confluence 500 Internal Service Error

[dimaliok]

Describe the bug
When updating a page using the UpdateContent() function, with content.MetaData unset, Confluence returns a 500 Internal Service Error with the following message in debug mode:
{"statusCode":500,"message":"java.lang.NullPointerException: Cannot invoke \"java.util.Map.size()\" because \"<parameter1>\" is null"}

Looking at the outputted request json, I see a "metadata":null field. It appears the confluence API doesn't handle null fields well, so there should probably be an "omitempty" in the Metadata struct.
To Reproduce
Steps to reproduce the behavior:
Send a curl request to the confluence api with the metadata field being null. Or use UpdateContent with the metadata field unset.

Expected behavior
Update to be successful instead of returning a 500 error.

Release version
1.4.6

Additional context
Possibly related to #59

[dimaliok]

Just wanted to add that a workaround is to set content.MetaData to an empty struct: content.Metadata = &goconfluence.Metadata{}