fix invalid unwrap when failing to create build file

[Techatrix]

getBuildFile may return null when creating the build file failed.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a6dbca2) 75.50% compared to head (f2c0e7c) 75.50%.

Files	Patch %	Lines
src/DocumentStore.zig	0.00%	1 Missing ⚠️
src/features/completions.zig	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1636   +/-   ##
=======================================
  Coverage   75.50%   75.50%           
=======================================
  Files          33       33           
  Lines        9034     9034           
=======================================
  Hits         6821     6821           
  Misses       2213     2213           

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[llogick]

I know there are a few more call sites, are they covered?

[Techatrix]

I know there are a few more call sites, are they covered?

I've looked again and realized that I've missed one call and one of my changes was unnecessary which also applies to all the other callsites of getBuildFile which I didn't edit because of the following guarantee:

zls/src/DocumentStore.zig

Lines 160 to 162 in a3145f6

	/// `DocumentStore.build_files` is guaranteed to contain this uri
	/// uri memory managed by its build_file
	associated_build_file: ?Uri = null,

[llogick]

Got it