Return empty arrays from TaxonomyServiceRemoteREST tag/category mapping

[jkmassel]

Follows the pattern from #25964.

Summary

• TaxonomyServiceRemoteREST's tag/category fetches call their success blocks — declared NSArray<RemotePostTag *> * _Nonnull / NSArray<RemotePostCategory *> * _Nonnull — with [self remoteTagsWithJSONArray:[responseObject arrayForKey:…]].
• arrayForKey: returns nil when the key is absent or JSON-null, and [nil wp_map:…] is nil, so a malformed 200 response leaks nil into the _Nonnull block.
• The Swift caller (TagsService, for .com sites) bridges that to non-optional [RemotePostTag] and traps at the closure boundary.

Fix

• Coalesce the two map helpers to @[] (… ?: @[]). A missing tag/category list semantically is empty, so the _Nonnull block contract stays true for every current and future caller. This covers all six tag/category fetch entry points at once (getTags*, searchTagsWithName:, getCategories*, searchCategoriesWithName:).

Test plan

• The WordPressKitObjC module builds via the WordPress app build (generic iOS Simulator).
• Tag and category lists still load for a .com site.

Part of the Objective-C non-null nullability audit; see #25964.

[wpmobilebot]

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	34125
	Version	PR #25969
	Bundle ID	org.wordpress.alpha
	Commit	50d24ca
	Installation URL	57js9e4ilv0ig

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	34125
	Version	PR #25969
	Bundle ID	com.jetpack.alpha
	Commit	50d24ca
	Installation URL	6d94raluft0qg

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.