Added a verification to ensure the group tables are not empty

[joris-ati4]

When trying to skip a configuration file without tables as stated in the docuemention https://github.com/elgentos/masquerade#customization. A warning is triggered

PHP Warning:  Invalid argument supplied for foreach() in masquerade.phar/src/Elgentos/Masquerade/Console/RunCommand.php on line 131

To prevent this and solve the issue https://github.com/elgentos/masquerade/issues/109, I added a verification to ensure tables is not empty