fix: dist option should be undefined if not specified

[armenzg]

In #125, we added support for using dist with sourcemaps, however, we return null when not specified and that gets converted into the string "null" down the pipeline which has broken the sourcemaps for various customers in the last two weeks.

Also, ss indicated in #138, dist is optional for sourcemaps when used with the CLI. Updating the README.me to handle it.

This also removes a warning which shows when the action is run:

Warning: Unexpected input(s) 'dist', valid inputs are ['entryPoint', 'args', 'environment', 'sourcemaps', 'finalize', 'ignore_missing', 'ignore_empty', 'started_at', 'version', 'version_prefix',
'set_commits', 'projects', 'url_prefix', 'strip_common_prefix', 'working_directory']

Fixes #140