expand macros for non-existant custom variable as empty string

Closes #983

Changelog

@@ -7,6 +7,7 @@ Nagios Core 4 Change Log
 * Send a recovery notification if the object recovered while flapping (#847) (Dylan Anderson)
 * Fix for separate build directory & a couple small cleanups (Doug Nazar)
 * Update tutorial links in Readme (Igor Udot)
+* Expand the custom variable macros to an empty string, if the custom variable does not exist
 
 4.5.5 - 2024-09-17
 ------------------

common/macros.c

@@ -2467,7 +2467,7 @@ int grab_custom_object_macro_r(nagios_macros *mac, char *macro_name, customvaria
 	customvariablesmember *temp_customvariablesmember = NULL;
 	int result = ERROR;
 
-	if(macro_name == NULL || vars == NULL || output == NULL)
+	if(macro_name == NULL || output == NULL)
 		return ERROR;
 
 	/* get the custom variable */
@@ -2484,6 +2484,12 @@ int grab_custom_object_macro_r(nagios_macros *mac, char *macro_name, customvaria
 			}
 		}
 
+	/* expand nonexistant custom variables as an empty string */
+	if( result == ERROR ){
+		*output = "";
+		result = OK;
+		}
+
 	return result;
 	}