Return missing for empty cells of type "s"

[ararslan]

For most other cell data types, we're returning missing if the cell value is empty. The check for emptiness comes after the call to sst_unformatted_string, but sst_unformatted_string calls parse(Int, _) on cell.value, which errors if cell.value is empty. Thus here I've inserted a check in keeping with other branches for isempty(cell.value).

I don't really know anything about the internals of this package (or about the XLSX format) and I'm not sure how to unit test this, but this fixes an issue I encountered.

[codecov]

Codecov Report

Merging #109 into master will decrease coverage by 0.05%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   95.02%   94.96%   -0.06%     
==========================================
  Files          13       13              
  Lines        1507     1509       +2     
==========================================
+ Hits         1432     1433       +1     
- Misses         75       76       +1

Impacted Files	Coverage Δ
src/cell.jl	89.01% <50%> (-0.88%)	⬇️

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[codecov]

Codecov Report

Merging #109 into master will decrease coverage by 0.05%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   95.02%   94.96%   -0.06%     
==========================================
  Files          13       13              
  Lines        1507     1509       +2     
==========================================
+ Hits         1432     1433       +1     
- Misses         75       76       +1

Impacted Files	Coverage Δ
src/cell.jl	89.01% <50%> (-0.88%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4481de0...0615915. Read the comment docs.