[FIX] Fix deprecated usage of imagestring() and implicit conversion from float to int in gd.php

• Updated the usage of imagestring() to avoid passing null to the $string parameter, addressing the Deprecated error by adding a check for issue:https://app.glitchtip.com/evoludata/issues/5862940?project=4783
• Corrected implicit conversion from float to int by rounding to the nearest integer for issue: https://app.glitchtip.com/evoludata/issues/5862942?project=4783