TemplateService should return all templates from GetAllAsync if no keys are passed

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

This is the TemplateService equivalent of #16230

To align functionality of our services, GetAllAsync(params Guid[] keys) must return all templates if no keys have been passed to the method.

This is functionally breaking, but "only" towards the new V14 service methods.

I have verified that all consumers of this method are guarded by an explicit check for zero results, thus ensuring we don't accidentally return all templates when we're in fact wanting to return none.

Testing this PR

GetAllAsync(params Guid[] keys) is only used by the template "item" and "search" endpoints. Both should still work 😄

• The "item" endpoint should return no templates if no item keys are passed.
• The "search" endpoint should return no templates if there are zero matches to the search query.
• The "search" endpoint should return no templates if paginating "past" the total number of results in a query that yields results.