lisa.platforms.platinfo: Fix exception when getting a key from trace metadata
lisa.platforms.platinfo: Fix exception when getting a key from trace metadata

If the metadata is not available, raise the exception (which will be
turned into a ConfigKeyError) rather than returning None and ending up
with a TypeError.

credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa