drivers: wifi: Prevent CMake warnings for out-of-tree drivers

When Ethernet or Wi-Fi subsystems are enabled, but no driver source
files are provided in the Zephyr source tree (which is the case for
out-of-tree drivers), CMake throws warnings about empty libs, for
example:

  No SOURCES given to Zephyr library: drivers__ethernet
  No SOURCES given to Zephyr library: drivers__wifi

This commit sets ALLOW_EMPTY property for those libraries, to allow for
seamless out-of-tree drivers integration.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>