ASoC: mediatek: sof-common: Add NULL check for normal_link string

In the Linux kernel, the following vulnerability has been resolved:

ASoC: mediatek: sof-common: Add NULL check for normal_link string

It's not granted that all entries of struct sof_conn_stream declare

a `normal_link` (a non-SOF, direct link) string, and this is the case

for SoCs that support only SOF paths (hence do not support both direct

and SOF usecases).

For example, in the case of MT8188 there is no normal_link string in

any of the sof_conn_stream entries and there will be more drivers

doing that in the future.

To avoid possible NULL pointer KPs, add a NULL check for `normal_link`.