[mlir][Tutorial] Make parsing an empty file print a better error.

[mlir][Tutorial] Make parsing an empty file print a better error.

Summary:
Previously, we would, for an empty file, print the somewhat confusing
  Assertion `tok == curTok [...]' failed.
With this change, we now print
  Parse error [...]: expected 'def' [...]

This only affects the parser from chapters 1-6, since the more advanced
chapter 7 parser is actually able to generate an empty module from an
empty file.  Nonetheless, this commit also adds the additional check to
the chapter 7 parser, for consistency.

Differential Revision: https://reviews.llvm.org/D75534