config.Config: only set x509userproxy in condor settings if one is provided

gstlal/python/config/__init__.py

@@ -128,8 +128,8 @@ class Config:
 			submit_opts['+SingularityImage'] = f'"{singularity_image}"'
 			submit_opts['transfer_executable'] = False
 			submit_opts['getenv'] = False
-			if self.condor.transfer_files:
-				submit_opts['x509userproxy'] = 'x509_proxy'
+			if self.condor.transfer_files and self.source.x509_proxy:
+				submit_opts['x509userproxy'] = self.source.x509_proxy
 				submit_opts['use_x509userproxy'] = True
 		else:
 			submit_opts['getenv'] = True