Previously we were treating the local `NullGrammar` differently from
`atom.grammars.nullGrammar`. These two grammars are conceptually the
same, as the former was created to support editors creation without a
grammar registry.
To keep backwards-compatibility, we also build placeholder lines
on-demand when calling `TokenizedBuffer.prototype.tokenizedLineForRow`.
This ensure that packages relying on the internals of `TokenizedBuffer`
to retrieve syntactic boundaries won't break when the null grammar is
used or large file mode is on.