Core: Avoid null references that may be returned due to concurrent changes or inconsistent cluster state

Closes #7181
Mappings: Make sure that multi fields are serialized in alphabetic order to ensure that the source is always the same.

Closes #7215
Test: don't need to wait for concrete mappings since we map both fields when we create the index
Internal: adjusted TermVectorRequest serialization to not serialize and de-serialize the index twice

Closes #7221