fix error when header is empty

catch case of empty header to not crash

build/utils/pytex_file.py

@@ -68,6 +68,7 @@ class TexFileToFormat:
 
     def __format_header(self):
         new_header = []
+        if self.current_build_info.header:
             for line in self.current_build_info.header:
                 new_header.append(line.format(
                     source_file=self.src_path.name,