log/slog: replace nil contexts with context.Background()

log/slog: replace nil contexts with context.Background()

Passing nil for a context is discouraged. We should avoid it.

Fixes #61219.

Change-Id: I664387070aacb56af580b6b0791ca40982d2a711
Reviewed-on: https://go-review.googlesource.com/c/go/+/508437
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>