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Skip param extraction for a route whose pattern captures nothing (#2879)

`Router#process_route` asks every matched route for its path params:

    routing_args = route.params_for(input) || {}

`params_for` hands the input to Mustermann, which runs the route's regexp and
builds a Hash of named captures, and Grape then walks that into a second Hash
keyed by Symbol.

A fully static path has no named captures to find. `/health`, an index route,
anything whose params all arrive as query or body — the compiled regexp names
nothing, not even `format`, which the suffix spells as a literal alternation.
The whole exchange runs the regexp a second time (the router's union already
matched it, which is how the route was chosen) to arrive at an empty Hash.

Resolve it once, when the pattern is compiled, and skip the round trip when
there is nothing to extract. The method answers nil, which is what
`GreedyRoute#params_for` already does for the same reason, and what the single
call site already coerces — the `|| {}` was there before this change.

On a static route (median of 9 interleaved runs, Ruby 4.0.5 with YJIT):

    throughput   233,443 -> 263,742 req/s   (+13.0%)
    allocations       25.0 -> 20.0 objects per request

Routes that do capture are unaffected beyond one ivar read. Nothing observes a
different value than it did before: the only caller turns nil into the Hash it
builds routing args in, exactly as it already did for greedy routes.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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