[MINOR][BUILD][FOLLOWUP] Fix merge_spark_pr script for no jira case

### What changes were proposed in this pull request?

This PR is a followup for #50237 to fix the case of `if not JIRA_IMPORTED:`

### Why are the changes needed?

fix merge tool

### Does this PR introduce _any_ user-facing change?
no, committer member only

### How was this patch tested?

merged #50443 w/o jira installed

### Was this patch authored or co-authored using generative AI tooling?

no

Closes #50447 from yaooqinn/jira.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Kent Yao <[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:
@@ -595,7 +596,6 @@ def initialize_jira():
     else:
         print("Neither JIRA_ACCESS_TOKEN nor JIRA_USERNAME/JIRA_PASSWORD are set.")
         continue_maybe("Continue without jira?")
-        asf_jira = None
 
 
 def main():