Fix for failure when pr body is None

as seen in https://github.com/secondlife/viewer/actions/runs/8207520150/job/22448969537#step:9:140

Author: brad-linden

which-branch/which_branch.py

@@ -98,6 +98,8 @@ def main(*raw_args):
 
     # pr.body is the PR's description. Look for a line embedded in that
     # description containing only 'relnotes:'.
+    if pr.body is None:
+        return
     lines = iter(pr.body.splitlines())
     try:
         next(line for line in lines if line.strip() == 'relnotes:')