Fix unexpected empty source map stacks

[jscheid]

Description

This fixes two unrelated issues that both previously presented with a similar symptom, namely the following error:

RangeError: cannot get the first element of beginless range

The first issue is that some constructs, such as {} => {} and foo in bar get parsed by Ripper as a case AST node. However, without a matching case keyword, the YARD parser doesn't know where it starts.

The second issue is that tokens following the def keyword are (correctly) never treated as keywords themselves, because they are used as the method name instead. However, this logic erroneously also applied to keywords following the :def symbol - again making the YARD parser unable to determine where the keyword (following the :def symbol) starts.

In both cases, the problematic constructs by themselves did not cause a crash but merely provided wrong source ranges. However, in specific parser states, the corresponding @map entry is the empty array rather than nil (because a similar, valid, construct was encountered before, which initializes the map entry). This was causing crashes since the beginning of line_range and source_range becomes nil as a result.

Here we're fixing the underlying issue and we're also adding specs to ensure that source ranges are correct, and that crashes in the presence of pre-initializing constructs are eliminated.

Note that another fix would have been to treat empty @map entries the same as nil @map entries in visit_event. Doing so would also prevent the crashes, and improve robustness more generally, however it would mask potential similar problems with other constructs. Therefore I have refrained from making that change -- a debatable decision that might be revisited in a separate change. However, I've added a better error message in this case.

DISCLOSURE: Claude wrote large parts of the specs and helped me understand the problems and create the fixes, but I'm fully adopting its output as my own (after polishing it manually and reviewing it.)

Fixes #1603.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).