Set initial values for animation and transition property.

[hiikezoe]

These properties are not allowed to set empty vector.
This is a follow up fix for #15761.
Also this fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1343168

r? @Manishearth

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #__ (github issue number if applicable).

• [] There are tests for these changes OR
• These changes do not require tests because I am not sure we should write test cases for this , anyway gecko has some test cases.

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @bholley: components/style/properties/shorthand/box.mako.rs
• @emilio: components/style/properties/shorthand/box.mako.rs

[highfive]

Warning

• These commits modify style code, but no tests are modified. Please consider adding a test!

[upsuper]

I think this is wrong. When animation is set to none, all its subproperties should be set to none as well. It doesn't make sense to have one "initial" value inside those subproperties.

[upsuper]

Actually I don't understand why we have an allow_empty in vector_longhand at all. Shouldn't all those vector longhands be empty when they are set to none?

[hiikezoe]

I guess the reason is that the comment for allow_empty was somewhat misleading.
#14913 (comment)

[Manishearth]

Actually I don't understand why we have an allow_empty in vector_longhand at all.

for the background/etc properties, you don't actually parse none as a separate keyword, each vector element is allowed to be none (you can have none, none, url(), none). But for other properties without none values in their individual vector elements, they need a global none value.

Parsing bg-foo as none | <bg-foo># where bg-foo itself is stuff | none would cause problems because the zero case would count as the one case and the number of elements here (even if null) is important. So I've kept these as separate cases with allow_empty