Fix Trace command when non valid files are found

[promatik]

Hi @jasonmccreary!

My last PR (#488) introduced a possible problem on Trace command, so I'm here to fix it!

We found that non valid files, like a .zip file, breaks the Trace command, it generates an empty entry with a backslash ("\"), and class_exists breaks with that string.

It's a rare situation, but when it happens developers have no feedback about it.
This PR fixes it.

[tabacitu]

@promatik any chance we can have a test for this? It would prove the problem AND make sure it doesn't break in the future... 👀

[promatik]

@tabacitu did it 👌

@jasonmccreary, I just added a full test to the Trace command.
It was not possible to test the changed function individually, because it's a private function, it makes only sense to test the whole Trace command.

Please note that Trace uses the database to retrieve each column information, now to run the tests one must have sqlite on their system and the extension must be installed and enable on php – it probably already is, any way here is the command;

sudo apt-get install php7.4-sqlite

[jasonmccreary]

Why wouldn't we limit to just .php extensions?

[promatik]

Did it, makes total sense 👌
And its a performance improvement because the file is not even read.