Added verifications before calling callbacks

[AssemblyJohn]

Describe your changes

Added checks for functors before an actual call to it.

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• If OCPP 2.0.1: I have updated the OCPP 2.0.1 status document
• I read the contribution documentation and made sure that my changes meet its requirements

[Pietfried]

Did you understand how it could happen that these callbacks are nullptr when they are called?

[AssemblyJohn]

No, I could not find any culprit in that case. An initial theory was that the callback captures the 'this' pointer and that somehow becomes stale, but it is improbable, since in that case we would probably have a segfault.