Don't require headers for vendor libraries if not supported by any built platform

[ThadHouse]

Currently, the headers artifacts MUST exist, even if the binaries for the only platform built don't exist, because the resolver still needs to see them.

https://github.com/wpilibsuite/GradleRIO/blob/master/src/main/groovy/edu/wpi/first/gradlerio/wpi/dependencies/WPINativeJsonDepRules.groovy#L45

[ThadHouse]

Also need to make sure this doesn't break header only libraries.