On empty input the awk script was always successful which caused the
filter-refcmp tests to always succeed.

Also fix the command lines for refcmp_metadata compare function because it
needs auto conversion filters, and update reference of test
filter-refcmp-psnr-rgb because it was missed in
a7fc78c1 but was never noticed due to the
original issue...
Signed-off-by: Marton Balint <cus@passwd.hu>