No break space character metrics

[adrg]

Use regular space character metrics for the no-break space character if nbsp handling is not already defined.
Addresses #396

[codecov]

Codecov Report

Merging #397 into v3 will increase coverage by 5.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##               v3     #397      +/-   ##
==========================================
+ Coverage   55.37%   60.56%   +5.19%     
==========================================
  Files         151      151              
  Lines       27435    27438       +3     
==========================================
+ Hits        15193    16619    +1426     
+ Misses      10380    10355      -25     
+ Partials     1862      464    -1398

Impacted Files	Coverage Δ
pdf/model/internal/fonts/std.go	73.8% <100%> (+4.57%)	⬆️
pdf/annotator/field_appearance.go	11.5% <0%> (+0.62%)	⬆️
pdf/creator/division.go	89.13% <0%> (+1.08%)	⬆️
pdf/model/outline.go	86.74% <0%> (+1.2%)	⬆️
pdf/creator/invoice.go	90.07% <0%> (+1.21%)	⬆️
pdf/model/sighandler/sighandler_rsa_sha1.go	6.75% <0%> (+1.35%)	⬆️
pdf/annotator/form_field.go	14.78% <0%> (+1.4%)	⬆️
pdf/creator/toc_line.go	75.39% <0%> (+1.58%)	⬆️
pdf/model/annotations.go	23.28% <0%> (+1.65%)	⬆️
pdf/internal/ccittfax/decode.go	94.98% <0%> (+1.75%)	⬆️
... and 94 more

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