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Avoid empty-hash merges on request hot paths (#2689)

Three `|| {}` patterns on the per-request path allocated an empty Hash
and handed it to a merge that produced a shallow copy of the left-hand
side — pure waste. Guard the merge when the right-hand side is absent
or empty instead.

When a route has no path placeholders (or none captured), `route.params`
returns an empty Hash (or nil). The old code did
`args.merge(route_params || {})`, allocating `{}` and then a new hash
identical to `args` on every matched static route.

`grape_routing_args` falls back to `{}` when env has no routing args.
`deep_merge!` on `{}` is a walking no-op. Skip the call entirely when
routing_args is empty.

`(body || {}).merge(key => representation)` — when no body is set
(the common case), this allocated `{}` only to merge one key into it.
Build the one-key hash directly.

process_route, no route_params (static path, common case):
  old: 4.48 M i/s, 480 objects allocated / 1k calls
  new: 8.31 M i/s, 160 objects allocated / 1k calls  (1.85x faster, 3x fewer)

process_route, empty {} route_params:
  old: 5.16 M i/s, 320 objects / 1k calls
  new: 8.02 M i/s, 160 objects / 1k calls  (1.55x faster, 2x fewer)

process_route, real route_params: within noise (unchanged path).

No behavior change; all 2,236 specs pass.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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