Update AbstractParameter.php

[d-buchmann]

Make lazy null conditionals explicit, thus enabling output of 0 as parameter value.
Try to handle LaTeX special chars gracefully (especially %, as seen in the referenced issue).
Fixes #958

(Javascript code will need some work too: the LaTeX controllers also don't handle special chars.
The parameter selection drop-down show some params incorrectly, e.g. Tolerance [%] shows as Tolerance[.
The parameter preview shows an error e.g. when entering %; With \%, it works.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.86%. Comparing base (c445359) to head (39b0e85).
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #959      +/-   ##
============================================
+ Coverage     59.73%   59.86%   +0.13%     
- Complexity     6036     6038       +2     
============================================
  Files           529      529              
  Lines         20588    20603      +15     
============================================
+ Hits          12298    12334      +36     
+ Misses         8290     8269      -21     

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[jbtronics]

I think it is not useful to escape all latex characters, as you then cannot use latex in units anymore at all (which was the whole point for using latex for formatting the units). For simple units you don't need it, but for something like $nV/\sqrt{Hz}$, latex is very useful.

I changed it in a way, that only the percentage sign gets escaped, as it is very unlikely that somebody wants to put a comment into a unit, and it will be more likely used as a percentage sign.

[d-buchmann]

Make perfect sense.
I still have one small edit though: % inside the regex needn't be escaped, but if the user already escaped the percent sign, it would result in \\%, which we don't want.
Therefore capture and replace an optional backslash before the percentage sign as well.