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`.