Update __init__.py - use of None in mapper parameter

[pickworthi]

When a caller passes a value of None for the mapper parameter, a TypeException is raised. Usage of None by a caller to signify no value given is quite common, so this makes the calls a bit more robust.

[pickworthi]

In response to:
#3

[github-actions]

Pull Request Test Coverage Report for Build 11128821342

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 97.732%

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Totals
Change from base Build 10843354555:	0.01%
Covered Lines:	431
Relevant Lines:	441

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💛 - Coveralls