emacs: Fix applying stickiness to the :notmuch-part property

Previously, we simply called pushnew to add :notmuch-part to the
front-sticky and rear-nonsticky text property lists.  This works if
these are nil or lists, but they can also have the value t, meaning
that all properties are front-sticky/rear-nonsticky.  In this case,
pushnew will signal an error because t is not a list.  We never set
these properties to t ourselves, but since we apply these property
changes over arbitrary renderer output, we have to deal with this
possibility.