ResourceLoaderSkinModule: Fix SkinStyles extending of known media queries

ResourceLoaderSkinModule: Fix SkinStyles extending of known media queries

If any of the styles given in its module definition (in the
'styles' or 'skinStyles' properties) used the same media queries
as the module's own CSS (e.g. 'all'), the module would fail with
"PHP Fatal error: [] operator not supported for strings" because
FileModule defaults to merging all the stylesheets into a single
string.

Fix this by ensuring they are arrays before trying to extend them.

This previously made it impossible to use $wgResourceModuleSkinStyles
for modules that use SkinModule (instead of plain FileModule), such as
the 'mediawiki.skinning.interface' module.

Bug: T168088
Change-Id: I3effcaa4982728e707fbf9efeec4e5e78fc8aab6