Make parseSharedStrings return optional value

[MaxDesiatov]

Resolves #121.

[codecov]

Codecov Report

Merging #122 into main will decrease coverage by 0.28%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
- Coverage   97.34%   97.05%   -0.29%     
==========================================
  Files           8        9       +1     
  Lines         301      306       +5     
==========================================
+ Hits          293      297       +4     
- Misses          8        9       +1     

Impacted Files	Coverage Δ
Sources/CoreXLSX/XLSXFile.swift	96.82% <85.71%> (-0.74%)	⬇️
Sources/CoreXLSX/Relationships.swift	100.00% <100.00%> (ø)

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