Fix unescaped "<" in MathML in PDF (#18520)
Link to issue number:
Fixes #18511
Summary of the issue:
If the structure tree is used for MathML in PDF, calls to getValue return the interpreted > so that it becomes < and we have an HTML syntax error.
Description of user facing changes:
The bug is fixed.
Description of developer facing changes:
None.
Description of development approach:
Called html.escape() for the result of the getValue (when it was not "none")
