Fix "hostName.empty()" bug and remove vid check

When going from C style to C++ style environment variable handeling, we must change some of the existence checks. In cta-verify-file the program will throw an error if HOSTNAME is not defined, as the empty() function checks if a string is equal to ""(c-style), but not nullptr. By removing the vid check, this bug is resolved.

In addition, the vid check should be removed, as it increased the run time when calling the tool with a single id. Run time will increase n^2 if a list of ids is not provided, but other tools do not use it efficiently. Therefore, a better solution to confirm that a vid exists must be found. This will be addressed in another issue. See the issue here #224 (closed)