fix(assert): treat a pointer to an empty interface as empty

[piyush295]

Fixes #1955.

Problem

isEmptyValue (rewritten in #1761, released in v1.11.0) regressed for a pointer to an interface. Since v1.11.0, Empty can never pass for a *SomeInterface and NotEmpty always passes for one, even when the pointee is unambiguously empty.

Root cause: the reflect.Ptr case recurses with objValue.Elem(). For a *I, Elem() yields a reflect.Value whose Kind() is reflect.Interface, which matched no case in the switch, so it fell through to return false.

The pre-v1.11.0 implementation used objValue.Elem().Interface(), which re-boxed through interface{}; reflect.ValueOf on the result collapsed the interface layer, so recursion continued into the concrete pointee.

Fix

Add a reflect.Interface case that recurses into the concrete value it holds, restoring the previous behaviour. A nil interface is already handled by the IsZero() fast-path at the top of the function.

Testing

• Added regression cases to Test_isEmpty (pointer to a nil interface, and a False case for a pointer to an interface holding a non-empty value).
• go test ./assert/ — full suite passes; go vet ./assert/ clean.