Empty Alt & Title Text field

Thank you very much for your response.
I actually used a multiple if conditions combined with strpos and strlen and managed to make it work for me in 99% situations.
I will write it here just in case someone has a similar problem.
In nextgen_basic_imagebrowser.php instead of:
<h3><?php echo esc_attr($image->alttext); ?></h3>
insert this:

<h3><?php
     $a = $image->alttext;
     if ((strpos($a,'-') !== false) or (strpos($a,'_') !== false)){
     } else {
     echo $a;
     }
     ?></h3>

This code will display all the Alt/Title Texts that you entered/edited in Manage Gallery and won’t display automatically created Alt/Title Texts (e.g. my-image-1.jpg) if you didn’t enter any Alt Text yourself.
It probably isn’t the most elegant solution and probably won’t work for everybody (depending on how you formatted your Titles) but it works for me.