Fix T56912: bpy.data.masks.new() crashed

Fix T56912: bpy.data.masks.new() crashed

I think there are two possible ways to fix that.
1. Make the name a required parameter.
2. Provide a default value.

I choosed option 1 in this fix to be consistent with other .new functions.

Also I think `RNA_def_string` instead of `RNA_def_string_file_path` should be used here. Looks like a copy-paste error.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D3728