fix signature help fail with invalid arg

[abap34]

I noticed that the language server throws an exception when the following typo occurs:

f(x; y = 1) = x + y
f(0; y.y=1│)

This appears to be syntactically valid (it fails during lowering), but signature-help seems to attempt to analyze it and fails. I added an additional check to handle this case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.72%. Comparing base (0a310ca) to head (b6b1d7a).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #582      +/-   ##
==========================================
+ Coverage   67.68%   67.72%   +0.04%     
==========================================
  Files          51       51              
  Lines        8309     8311       +2     
==========================================
+ Hits         5624     5629       +5     
+ Misses       2685     2682       -3     

Flag	Coverage Δ
JETLS.jl	67.72% <100.00%> (+0.04%)	⬆️

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