dev/core#227 Fix issue where on some extened groups the multiple reco…

[seamuslee001]

…rds checkbox doesn't show this causes empty string to be passed in which was causing problems when working out if we needed to make changes to the custom value table

Overview

When submitting the custom group setting form for custom groups extending activities the is_multiple checkbox doesn't show this causes an empty string to be passed through or is_multiple. This is replicated through the API. The test fails without the patch

Before

Hard fail when trying to edit the custom group settings for any custom groups extending activities

After

no hard fail

ping @eileenmcnaughton @monishdeb

[civibot]

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[eileenmcnaughton]

Looks good to me - @jmcclelland ....

[seamuslee001]

Merging as per the tag

[jmcclelland]

Just tested and it works for me. Thank you!