Allow falsy targets to be passed to the serializer

[rubensworks]

While updating rdflib from 0.12.x to 0.17.x in solid-permissions, I noticed that a bunch of unit tests in solid-permissions fail with the following error:

(node:12527) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'uri' of null
    at Object.serialize (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/node_modules/rdflib/lib/serialize.js:14:25)
    at Promise (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/src/permission-set.js:807:11)
    at new Promise (<anonymous>)
    at PermissionSet.serialize (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/src/permission-set.js:806:12)
    at Test.<anonymous> (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/test/unit/permission-set-test.js:245:13)
    at Test.bound [as _cb] (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/node_modules/tape/lib/test.js:76:32)
    at Test.run (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/node_modules/tape/lib/test.js:95:10)
    at Test.bound [as run] (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/node_modules/tape/lib/test.js:76:32)
    at Immediate.next [as _onImmediate] (/Users/rtaelman/Documents/UGent/nosync/solid/solid-permissions/node_modules/tape/lib/results.js:71:15)
    at runCallback (timers.js:810:20)

This crash occurs because the serialize.js assumes that target is defined, while solid-permissions sometimes passes null. Not sure who is at fault here, but in this PR I assume that rdflib should accept falsy target values.

With this change, all unit tests from solid-permissions pass on rdflib 0.17.x.