fix: nil-pointer panic from custom type function

A DecodeCustomTypeFunc returning nil to mean "leave this pointer
field unset" panicked in reflect.Value.Set, since reflect.ValueOf(nil)
is the invalid zero Value. Guard against a nil return before Set.

This is the panic from #68/#69: the earlier fix routed pointer fields
through the generic Ptr recursion instead of guarding nil, which
discarded a custom function's own populated pointer result and had
to be reverted. Guarding val == nil directly fixes the panic without
touching the case that broke.

decoder.go

@@ -199,6 +199,14 @@ func (d *decoder) setFieldByType(current reflect.Value, namespace []byte, idx in
 					return
 				}
 
+				// A custom function returning a nil interface has no value to
+				// set, eg. to signal "leave this field at its zero value" for
+				// an empty input; reflect.ValueOf(nil) is the invalid zero
+				// Value and Set would panic on it.
+				if val == nil {
+					return
+				}
+
 				v.Set(reflect.ValueOf(val))
 				set = true
 				return