cargo clippy: fix len_zero

[spl]

Fixes len_zero

[codecov]

Codecov Report

Merging #340 into master will decrease coverage by 1.12%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #340      +/-   ##
=========================================
- Coverage   40.73%   39.6%   -1.13%     
=========================================
  Files         135     135              
  Lines       60483   59482    -1001     
=========================================
- Hits        24635   23560    -1075     
- Misses      35848   35922      +74

Impacted Files	Coverage Δ
src/engines/spx2html.rs	0% <0%> (ø)	⬆️
tectonic/xetex-xetex0.c	77.36% <0%> (-2.18%)	⬇️
tectonic/xetex-ini.c	86.96% <0%> (-2.04%)	⬇️
tectonic/xetex-math.c	70.73% <0%> (-1.65%)	⬇️
src/config.rs	6.41% <0%> (-0.93%)	⬇️
src/io/itarbundle.rs	0% <0%> (ø)	⬆️

---

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[codecov]

Codecov Report

Merging #340 into master will decrease coverage by 1.12%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #340      +/-   ##
=========================================
- Coverage   40.73%   39.6%   -1.13%     
=========================================
  Files         135     135              
  Lines       60483   59482    -1001     
=========================================
- Hits        24635   23560    -1075     
- Misses      35848   35922      +74

Impacted Files	Coverage Δ
src/engines/spx2html.rs	0% <0%> (ø)	⬆️
tectonic/xetex-xetex0.c	77.36% <0%> (-2.18%)	⬇️
tectonic/xetex-ini.c	86.96% <0%> (-2.04%)	⬇️
tectonic/xetex-math.c	70.73% <0%> (-1.65%)	⬇️
src/config.rs	6.41% <0%> (-0.93%)	⬇️
src/io/itarbundle.rs	0% <0%> (ø)	⬆️

---

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 39cf0ce...4120d3e. Read the comment docs.