bytes: make TrimSpace return nil on all-space input

Issue #29122 introduced a subtle regression due to the way that
TrimFuncLeft is written: previously TrimSpace returned nil when given
an input of all whitespace, but with the #29122 changes it returned an
empty slice on all-space input.

This change adds a special case to the new, optimized TrimSpace to go
back to that behavior. While it is odd behavior and people shouldn't be
relying on these functions returning a nil slice in practice, it's not
worth the breakage of code that does.

This tweak doesn't change the TrimSpace benchmarks significantly.

Fixes #31038

Change-Id: Idb495d02b474054d2b2f593c2e318a7a6625688a
Reviewed-on: https://go-review.googlesource.com/c/go/+/169518
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bytes/bytes.go

@@ -788,6 +788,11 @@ func TrimSpace(s []byte) []byte {
 	// At this point s[start:stop] starts and ends with an ASCII
 	// non-space bytes, so we're done. Non-ASCII cases have already
 	// been handled above.
+	if start == stop {
+		// Special case to preserve previous TrimLeftFunc behavior,
+		// returning nil instead of empty slice if all spaces.
+		return nil
+	}
 	return s[start:stop]
 }