Remove empty element in list

Otherwise get a `null` when convert to some backend (es-rule,...)

rules/network/zeek/zeek_dce_rpc_printnightmare_print_driver_install.yml

@@ -13,7 +13,6 @@ references:
     - https://github.com/SigmaHQ/sigma/blob/master/rules/network/zeek/zeek_dce_rpc_mitre_bzar_persistence.yml
     - https://old.zeek.org/zeekweek2019/slides/bzar.pdf
     - https://www.crowdstrike.com/blog/cve-2021-1678-printer-spooler-relay-security-advisory/
-    - 
 tags:
     - attack.execution
     - cve.2021-1675
@@ -42,4 +41,4 @@ fields:
     - endpoint
     - named_pipe
     - uid
-status: stable
+status: stable