Demo: getSecure TeX see MathSearch

* Keep the tex output of texvc and treat png and tex file in the same way
* Fix: emtpty tex string on load from database
* new Method: MathTexvc::getSecuredTex
* partially fix: Broken tests

The failing tests are probably related to
Bug: 45194

Change-Id: Iaaf5c7ae018eeb0d014bbc2e06c7ffa7f40e14dc