Fix crash when the file name being statted is just "\n"

[kislaykishore]

Description

GCSFuse crashes when a stat call is made for a path where the file name is just "\n". Instead, return an ENOENT error.

Link to the issue in case of a bug fix.

b/391795883
GH Issue: #2939

Testing details

1. Manual - NA
2. Unit tests - NA
3. Integration tests - Added.

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 75.63%. Comparing base (d45540f) to head (d60dd26).
Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/fs/fs.go	30.76%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2933   +/-   ##
=======================================
  Coverage   75.62%   75.63%           
=======================================
  Files         125      125           
  Lines       17641    17656   +15     
=======================================
+ Hits        13341    13354   +13     
  Misses       3738     3738           
- Partials      562      564    +2     

Flag	Coverage Δ
unittests	75.63% <30.76%> (+<0.01%)	⬆️

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