[DWARFVerifier] Fix verification of empty line tables (#81162)

A line table whose sole entry is an end sequence should not have the
entry's file index verified, as that value corresponds to the initial
value of the state machine, not to a real file index. In DWARF 5, this
is particularly problematic as it uses 0-based indexing, and the state
machine specifies a starting index of 1; in other words, you'd need to
have _two_ files before such index became legal "by default".

A previous attempt to fix this problem was done [1], but it was too
specific in its condition, and did not capture all possible cases where
this issue can happen.

[1]: https://github.com/llvm/llvm-project/pull/77004