[MINOR][INFRA] Fix merge_spark_pr script for no jira case

### What changes were proposed in this pull request?
Port apache/spark@be448f2

### Why are the changes needed?
Fix the merge tool

### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?

via Spark side

### Was this patch authored or co-authored using generative AI tooling?
no

Closes apache#70 from yaooqinn/yaooqinn-patch-1.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: yaooqinn

dev/merge_spark_pr.py

@@ -566,6 +566,7 @@ def get_current_ref():
 
 def initialize_jira():
     global asf_jira
+    asf_jira = None
     jira_server = {"server": JIRA_API_BASE}
 
     if not JIRA_IMPORTED: