Returning null still successfully adds entry

[campaignium]

Describe the bug
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To Reproduce
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See TroubleShooting LazyCache for an example of a minimal reproducable example.

Expected behavior
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** Framework and Platform

• OS: [e.g. Windows Server 2019]
• Framework [e.g. netcoreapp2.1]
• LazyCache Version [e.g. 2.0.3]

Additional context
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