Fix mailing task when there are no tasks

[mattwire]

Overview

Same fix for mailings as we did for activities here #20951

Before

Expanding "Mailings" on advanced search gives network error if there are no mailing action tasks.

After

Expanding "Mailings" on advanced search works if there are no mailing action tasks.

Technical Details

Comments

@eileenmcnaughton

[civibot]

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[eileenmcnaughton]

I had an issue replicating this - the 'print' task doesn't seem to show for me on the screen I was trying to replicate it on - even though it should.

However, I was able to replicate the error & the fact this fixed it in the end - and the fact that I see no actions ever on this page seems unchanged by this PR - so I'm merging (& quite possibly ignoring )