[PD] Fix isFeatureMovable() to check null status of property
[PD] Fix isFeatureMovable() to check null status of property

As reported by @donovaly, if you try to move a pipe that does not have
an auxilliary spine, the code segfaults. This commit adds a check to ensure
that not only does the property exist (which in this case it always does), but
also that it does not contain a null. That is a valid value for the property
to have, but cannot be itself interrogated for moveability by the isFeatureMovable
recursive call. This fix is also applied to the other similar conditions in that
function to ensure they never yield the same segmentation fault.

Forums topic: https://forum.freecadweb.org/viewtopic.php?p=479388#p479388

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