[12.x] fix: continue route matching rather than returning second fallbackRoute

[ryzr]

PR #57871 introduced a very slight change to the behaviour of matching fallback routes.

Before #57871: Return first non-fallback route match, otherwise return first fallback route match.

After #57871: Return the second matching fallback route (if one exist), otherwise return the first non-fallback route match, otherwise return the first fallback route.

Reasoning:
After the first fallbackRoute is matched, the next fallbackRoute will no longer get caught by the ->isFallback guard, due to the === null check. This causes the next matching fallbackRoute to be returned, rather than continuing through the non-fallback routes.

To fix this, I've removed the === null check, causing the loop to continue. I used null coalescing assignment to ensure only the first fallback route is captured.

[bytestream]

Noticed this too and this fixes it 👍🏼 . Think a test would be useful to prevent future regression.