fix(runtime-core): unable to set object to dom props

[ycmjason]

This PR is an alternative to #1092. #1092 probably is quite expensive to do, since a new element is created per prop just to check if that prop is string assignable.

This PR removes casting nullish values to '' for props which I personally think will make more sense because user can expect default browser behaviour for setting <element>.prop.

fix #1049

[blake-newman]

To me this makes the most sense, the null casting to an empty string seems to be a convenience layer only.

However, this casting can be done in userland when desired, which firstly fixes #1049 and as the alternatives are computational heavy we can remove from the framework.