escapeString: set string to 0

fix for influxDB write error. 'null' is not a valid number for value= field

Author: schulzad

smartapps/influxdb-logger/influxdb-logger.groovy

@@ -805,7 +805,7 @@ private escapeStringForInfluxDB(str) {
         //str = str.replaceAll("'", "_")  // Replace apostrophes with underscores.
     }
     else {
-        str = 'null'
+        str = '0'
     }
     return str
 }