fix: insufficient label checks for validity + compare tweaks

[TateB]

changes:

• added checkValidLabel calls for labels fetched via ens.nameByHash
  • i was previously under the impression that the rainbow table didn't have labels with ./[/], but it does
• removed all === for ==, and !== for !=
  • strict equality is compiled differently in older assemblyscript versions (being a comparison of memory pointer rather than data). not sure why graph-ts/graph-cli don't use newer versions yet (using 2+ year old v0.19.10), in newer versions this is fixed.
• replaced type definition for isNullable so typescript lint doesn't get annoyed by the lack of type narrowing
• resolvers are now always checked for their existence in ResolverEvent handlers to ensure a non-nullable value isn't null