Fixed crashes on unresolved template declarations

[artemrizhov]

Summary

Prevent null dereferences while processing template specializations whose
TemplateName cannot be resolved to a TemplateDecl.

This affects both class and sequence diagram visitors when handling
template-template parameters such as C<T>.

The change:

• returns without adding an unresolved class relationship;
• skips unresolved class-template method calls in sequence diagrams;
• reuses the validated template declaration instead of resolving it again.

Regression test

Adds t00102, covering both visitors with a template-template parameter:

• a class method parameter of type C<T>;
• a call to C<T>::call().

Before this change, these cases can dereference the null result of
getAsTemplateDecl() and crash.

[bkryza]

@artemrizhov Thanks for the PR - could you also add an entry in the test_cases.yaml - just after the entry for t00101 - https://github.com/bkryza/clang-uml/blob/master/tests/test_cases.yaml#L302, just put some title and description can be empty...

[artemrizhov]

Done