Fix located URI merging in action.merge_record_assets()

Fix located URI merging in action.merge_record_assets()

Perhaps due to the unexpected results of the XPath expression, which
returns a count of nodes for subfields, etc, rather than just a count of
the number of 856 datafields, NULL was being concatenated onto a string
- which of course made the string NULL.

Use COALESCE() to prevent the NULLness from spreading and enable
action.merge_record_assets() to do the right thing.

Signed-off-by: Dan Scott <dscott@laurentian.ca>