testing/checkers: fix panic in IsTrue

Fixes a panic caused by not handling an empty interface value, the zero value for interface{} properly.

This came up with a test case that tried

    c.Assert(m["somekey", jc.IsTrue)

Which would panic if "somekey" was missing from the map.
Merge pull request #34 from juju/100-fix-panic-testing-zero-interface-value-to-bool

testing/checkers: fix panic in IsTrue