Keep returning nil for the cases where we historically returned nil,
even though this is slightly different for TrimLeft and TrimRight.
Fixes #51793
Change-Id: Ifbdfc6b09d52b8e063cfe6341019f9b2eb8b70e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/393876
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>