Bugfix: mock existing local function.

[nestoracunablanco]

The mock is not applicable in case the function already exists and the specified closure is null.

Link to the issue: #461

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[nre-ableton]

Please fix the commit message so that the subject describes the change actually being made. When looking at the commit in git log, it should be clear to the reader what is happening without needing to navigate to GitHub and look up a closed issue.

The commit message body is fine and can be left as-is.

[nestoracunablanco]

@nre-ableton the commit message is changed. Thanks for reviewing!