fix: avoid error on empty command lines

merge_srt.py

@@ -57,6 +57,8 @@ class SRTMerger:
         }
         for line_number, line in enumerate(lines, start=1):
             parts = SRTMerger.split_line(line)
+            if not parts:
+                continue
             command = parts[0].upper()
             if command not in supported:
                 errors.append(f"Line {line_number}: unknown command '{command}'.")