handle panic in (*ptr)nil cases

[adamglt]

Value translation panics for nil interface{}.
Similar to what's described here: https://golang.org/doc/faq#nil_error

For example, with this combo:

var niltime *time.Time
params := M{"props": M{"name": "hello", "bla1": niltime}}
q := `
CREATE (b: Bla $props)
RETURN properties(b)
`

You get a panic: reflect: call of reflect.Value.Interface on zero Value.

This is a pretty common case when mapping from domain structs to map[string]interface{}.

[ali-ince]

Hi @adamglt, thanks for submitting this PR. To be able to merge your changes we will need a signed CLA from you. See http://neo4j.com/developer/cla/ for further information. Additional information on contributing code is found at http://neo4j.com/developer/contributing-code/.

Thanks.

[adamglt]

Sent, thanks.