dx(runtime-core): warn when the prop type is []

[skirtles-code]

A common mistake when declaring props is to use type: [] instead of type: Array. The mistake leads to a somewhat cryptic warning message:

Invalid prop: type check failed for prop "apple". Expected , got Array

SFC Playground

This PR introduces special handling for the empty array, so that it yields a more specific warning:

Prop type [] for prop "apple" won't match anything. Did you mean to use type Array instead?

Example

In theory it is possible to perform this check much sooner, rather than waiting for prop validation to fail, but I didn't think it was worth the extra complexity.