pw_symbolizer: Support mypy --no-implicit-optional
pw_symbolizer: Support mypy --no-implicit-optional

Change-Id: Id3056ff85c8131e763126addd13c46230e903c13
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/115210
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Reviewed-by: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed