imera: protect against NPE in FsSqlDriver#read

    RB: https://rb.dcache.org/r/7385/
    Acked-by: Paul Millar
    Target: trunk
    Request: 2.11 2.10 2.9 2.8 2.7 2.6
(cherry picked from commit f1252364cceec877505eb98a736abcc984bc964e)