Fixing the error caused by null elements in array

[semihbkgr]

fixes #296

It was failing when working on an array which has null elements. It is basically caused by trying to set the reflect.Value of the nil pointer to slice index.

panic: reflect: call of reflect.Value.Set on zero Value

goroutine 1 [running]:
reflect.flag.mustBeExportedSlow(0x1517da0?)
	reflect/value.go:235 +0xc5
reflect.flag.mustBeExported(...)
	reflect/value.go:229
reflect.Value.Set({0x1517da0?, 0xc0002a8620?, 0x7?}, {0x0?, 0x0?, 0xc0002b1790?})
	reflect/value.go:2155 +0x9f
github.com/tomwright/dasel/v2.derefSlice({0x1517da0?, 0xc000267cb0?, 0xc0001f4a20?})
	github.com/tomwright/dasel/v2/value.go:347 +0x1ad
github.com/tomwright/dasel/v2.deref({0x1517da0?, 0xc000267cb0?, 0x0?})
	github.com/tomwright/dasel/v2/value.go:370 +0x65
github.com/tomwright/dasel/v2.derefMap({0x1517da0?, 0xc00025ae60?, 0xc0002b1901?})
	github.com/tomwright/dasel/v2/value.go:359 +0x13d
...

an example of how it works after the changes:

[
  {
    "a": "alice",
    "b": 9,
    "c": [
      null,
      "bob",
      13,
      null,
      9,5,
      null
    ]
  },
  null
]

$ dasel -r json -w yaml -f test.json

- a: alice
  b: 9
  c:
  - null
  - bob
  - 13
  - null
  - 9
  - 5
  - null
- null

[TomWright]

Thanks for the contribution 👏