CI: Handle empty FVP_PV in the FVP report

In the python FVP report script, handle image builds which don't have a FPV_PV
value and those which don't have artifacts in the modules. Such builds should
be present within the report but with a blank space for the FVP_PV. In order
to show such blank lines, when the images are joined with the builds they are
merged on FVP_PV with a right join on the images rather than inner (which
means that the images with no matching build are still retained).

Signed-off-by: Debbie Martin <Debbie.Martin@arm.com>