Avoid a deprecated error from PHP8.1

[barto74]

Error "strtr(): Passing null to parameter #1 ($string) of type string is deprecated" avoided by converting value to a string.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #573 (6c3db76) into main (a336b53) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #573   +/-   ##
=======================================
  Coverage   73.00%   73.00%           
=======================================
  Files         128      128           
  Lines       23108    23108           
=======================================
  Hits        16870    16870           
  Misses       6238     6238           

Flag	Coverage Δ
php-5.3-ubuntu-latest	72.50% <100.00%> (ø)
php-5.4-ubuntu-latest	72.49% <100.00%> (-0.01%)	:arrow_down:
php-5.5-ubuntu-latest	72.49% <100.00%> (-0.01%)	:arrow_down:
php-5.6-ubuntu-latest	72.54% <100.00%> (ø)
php-7.1-ubuntu-latest	72.93% <100.00%> (-0.01%)	:arrow_down:
php-7.2-ubuntu-latest	72.81% <100.00%> (-0.01%)	:arrow_down:
php-7.3-ubuntu-latest	72.81% <100.00%> (ø)
php-7.4-ubuntu-latest	72.81% <100.00%> (ø)
php-8.0-ubuntu-latest	72.81% <100.00%> (-0.01%)	:arrow_down:
php-8.1-ubuntu-latest	72.81% <100.00%> (-0.01%)	:arrow_down:
php-nightly-ubuntu-latest	72.81% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
include/tcpdf_static.php	53.21% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.