extra_dhcp_opt add checks for empty strings

When passing extra-dhcp-opt into the port-create where an empty string
is provided as opt_value='   ', the create and update routine will load the
empty string into the DB. The result when written to the opts file is:
"tag:ece4c8aa-15c9-4f6b-8c42-7d4e285734bf,option:server-ip-address", which
when read by dnsmasq has been tested to cause dnsmasq to segment fault.

Change-Id: I31de4a3d27092bb219d20221c5ef5a6b22e050dc
Closes-Bug: #1257467
Merge "extra_dhcp_opt add checks for empty strings"