fix: Missing footage didn't report error

[vladavoX]

[vladavoX]

its okay to leave it as first unused prop, we will probably need it in future

[danixeee]

In After Effects ExtendScript, item.file can be null in cases where the FootageItem doesn't have an associated file source, such as:                          
                                                                                                                                                                
  1. Solids - Color solids created within AE have no file                                                                                                       
  2. Placeholders - Placeholder footage items                                                                                                                   
  3. Some generated content - Certain procedurally generated footage                                                                                            
                                                                                                                                                                
  The item.footageMissing property is true when the footage was linked to a file but that file can no longer be found on disk.                                  
                                                                                                                                                                
  So the two checks cover different scenarios:                                                                                                                  
  - footageMissing → file was linked but is now missing from disk                                                                                               
  - file === null → item never had a file source (solids, placeholders)                                                                                         
                                                                                                                                                                
  Both checks are needed if you want to catch all cases where you can't access actual file data. However, for the purpose of reporting "missing footage" to     
  users, you might want to distinguish these cases - a solid with file === null isn't really "missing" footage in the traditional sense.                        
                                                                                                                                                                
  If you only want truly missing footage (files that should exist but don't), item.footageMissing alone might be sufficient. But if file === null items would   
  cause issues downstream (e.g., when trying to access item.file.fsName), the null check prevents runtime errors.    

claude