pw_log: Avoid adding trailing commas

Do not list the format string as a separate argument in log macros to
avoid adding a trailing comma when the format string takes no arguments.

Change-Id: I57b419e704f2c804b5a0d9ced2702e666436bbb1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/71420
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>