srm: fix deserialise of TExtraInfo

The SRM saves the content of TExtraInfo for COPY requests, but fails
to deserialise it correctly if the user supplied no TExtraInfo.
This patch fixes that.

Target: master
Request: 2.10
Requires-book: no
Requires-notes: no
Acked-by: Tigran Mkrtchyan
Patch: https://rb.dcache.org/r/7147/