Make ve.dm.MWTemplateModel#serialize ignore empty parameters if they were not
present in the transclusion before the edit. This avoids dirty diffs where an
user edits a template transclusion via VisualEditor, and the editor adds all
available template parameters to the edit wikitext, even if they were not
changed during the edit.
This logic was ported from the old Wikia-WMF VisualEditor project.[1]
Additionally, add tests for ve.dm.MWTemplateModel serialization.
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[1] https://github.com/Wikia/app/pull/6450/commits/858eaa9
Bug: T101075
Change-Id: I35f8812724658904d30034db4e4684193a661c1e
