[BUGFIX] Use notEmpty rather than bool to determine sorting

Previously, when sortProperties is set to an empty array, SortableMixin will
attempt to sort the arrangedContent. With no sortProperties, orderBy() always
returns 0 when comparing two objects. Returning 0 is unstable in some browsers
(I.E. Chrome for arrays with length > 22), meaning the arrangedContent might be
ordered unexpectedly.

Instead, use computed.notEmpty to set the isSorted property. An empty array will
no longer be sorted. When programmatically manipulating sortProperties, it is
easier to maintain sortProperties as an array, rather than setting to null when
empty.

Fix minor typo in test: rearrenge => rearrange