data: Add NilAt method to Field and Frame

[kylebrandt]

What this PR does / why we need it:
Adds the NilAt(i int) bool method to Field.

// NilAt returns true if the element at index idx of the Field is nil.
// This is useful since the interface returned by At() will not be nil
// even if the underlying element is nil (without an type assertion).
// It will always return false if the Field is not nullable.
// It can panic if idx is out of range.
func (f *Field) NilAt(idx int) bool {
	return f.vector.NilAt(idx)
}

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[gabor]

LGTM 👍