Reverted 'if' expression to be compatble with Ubuntu 18.04

Author: razman786

TigGUI/Images/Manager.py

@@ -624,7 +624,8 @@ def trimarray(array):
                 # restore card comments
                 for card in template.fits_header.keys():
                     if card in _header:
-                        if com := template.fits_header.comments[card]:
+                        com = template.fits_header.comments[card]
+                        if com:
                             _header.set(keyword=card, comment=com)
 
                 # set new header