fix(runtime-dom): properly handle custom style properties with falsy values (fix: #5322)

[LinusBorg]

style definition objects that had an initially falsy value for a custom property would add that custom property with the stringified falsy value:

<div :style="{ '--color': undefined }">

<!-- was rendered as: -->
<div style="--color: undefined;"> 

Note that this only affected initial values. if the falsy value was added in an update, the property was properly removed.

After this PR, the custom property would not be added in the first place on mount:

<div style>

close: #5322

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[yyx990803]

We should only treat null and undefined as removal (this is consistent with Vue 2 where false is rendered as-is) so I removed the check for false.