Serialiser_Engine: added null check in MethodBaseSerialiser

[alelom]

Issues addressed by this PR

Closes #2823

Added a null check in MethodBaseSerializer to avoid it throwing an exception during startup (plugin loading) as explained in #2823.

Test files

See #2823. I can provide an assembly that throws the error if deemed necessary.

Changelog

• Added a null check in MethodBaseSerializer.

[FraserGreenroyd]

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[FraserGreenroyd]

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[FraserGreenroyd]

This adds the braces that @adecler requested on the previous iteration of this PR, so this is good to go now for beta.

[FraserGreenroyd]

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