Fix escape for undefined variables

[Gerrit-K]

If the escape filter is used for an undefined variable (e.g. r'\VAR{foo|e}' with template.render()), latex.escape:61 will raise an error: TypeError: expected string or bytes-like object. This PR should fix it.

[P1robot]

Automated benchmark results (pass #0): passed

concurrency: 32
requests per second: 6160
concurrency: 128
requests per second: 7641
concurrency: 512
requests per second: 4685
concurrency: 1024
requests per second: 4006

[mbr]

Guess that brings it in line with \VAR{name}. Merging, thanks.