Fix endpoints that only differ by a trailing variable

Some of the endpoints are differentiated simply by lack of a variable on the end. This is bad because you get into the question of what to do when you have the following endpoint definitions:

• /_matrix/some/endpoint
• /_matrix/some/endpoint/$var

and you get a request that looks like:

/_matrix/some/endpoint/

Does this match the first definition? Or the second definition with $var = ""? To make matters worse sometimes "" is a valid value!

This is currently noticed in the following endpoints, but there may be more:

Specced (as of this issue):

• GET /_matrix/client/r0/rooms/{roomId}/state/{eventType}/{stateKey}
• GET /_matrix/client/r0/rooms/{roomId}/state/{eventType}

• GET /_matrix/client/r0/user/{userId}/filter/
• GET /_matrix/client/r0/user/{userId}/filter/{filter}

Unspecced (as of this issue):

• GET /_matrix/federation/v1/groups/{groupId}/categories/
• GET /_matrix/federation/v1/groups/{groupId}/categories/{categoryId}

• GET /_matrix/federation/v1/groups/{groupId}/roles/
• GET /_matrix/federation/v1/groups/{groupId}/roles/{roleId}

Start by reviewing the Matrix client and federation endpoint definitions listed in the issue, comparing each pair that differs only by a trailing variable or slash. Check whether other endpoint pairs have the same ambiguity; done means the affected definitions consistently distinguish the no-variable and variable forms without leaving the empty-value behavior unclear.