Fix crash when invoking resolve_constant with an empty constant name

[vinistock]

We were crashing when invoking resolve_constant with an empty constant name or an incomplete name like Foo::. This PR ensures that we don't crash.

I considered raising an error instead of returning nil, but I talked myself out of doing so because then we'd essentially set the expectation that the API validates the fully qualified name. For example, "" would raise, but without extra changes #@@<Foo> would be accepted. Instead of trying to validate every possible input, I just went with nil.

[vinistock]

• Simplify nesting_stack_to_name_id name creation #750
• Fix singleton name creation for nesting #749
• Fix crash when invoking resolve_constant with an empty constant name #746 👈 (View in Graphite)
• main

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