Add guards to severity mixin converter functions

In severity mixin when severityFromStringToCode() function gets and
empty input, it can return an error when calling .toLowerCase() on it.
This commit fixes it by adding a check before.

Author: gulyasgergely902

web/server/vue-cli/src/mixins/severity.mixin.js

@@ -22,6 +22,9 @@ export default {
     },
 
     severityFromStringToCode(severity) {
+      if (!severity) {
+        return -1;
+      }
       switch (severity.toLowerCase()) {
       case "unspecified":
         return Severity.UNSPECIFIED;