Normally when we interpolate values for CSS properties we convert empty values
(such as the empty "from" value created for a by-animation) to a suitable zero
value. However, when we use discrete interpolation that step never occurs and in
some rare cases that means we end up setting the empty value on the target
attribute directly which will have an incorrect result (e.g. setting "" for the
height property will have no effect, whereas setting "0px" will).
This patch makes us perform the empty value to zero value conversion when
performing discrete animation.
Unfortunately it is not possible to test this behavior with shorthands since
there are currently no shorthand properties that are additive.
MozReview-Commit-ID: JFT1tis1RAR
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extra : rebase_source : cc444b6d4d5571c8ab231d88c4d349ea0713baaa
