emul: Don't panic if matching emul can't be found

When initializing emulators for devices registered on an emulated bus,
Zephyr will assert if a matching emulator for the device cannot be
found. This feels overly restrictive --there may be cases where we still
want to build a driver for testing even without an emulator and drivers
should be able to handle situations where there is no device emulator
present (the I2C/SPI transactions will simply fail and the driver never
becomes ready).

This commit removes the assert and replaces it with an warning message if
no matching emulator is found.

Signed-off-by: Tristan Honscheid <honscheid@google.com>