fix nullable nested list variable coercion

[jbellenger]

Hello again! I'm the maintainer of graphql-conformance, which verifies spec conformance across the GraphQL ecosystem.

I noticed that gqlgen had a test failure that appears to originate in gqlparser.

The issue is that gqlparser does not handle valid null values when coercing nested lists.

For example, given this valid value for a type like [[InputType!]]:

[
  [{ "name": "foo" }],
  null
]

The second element is a nullable inner list. gqlparser currently unwraps that null into an invalid reflect.Value, then panics when it tries to inspect the value's type:

reflect: call of reflect.Value.Type on zero Value

This PR fixes this by checking whether a reflected value represents null before trying to coerce it as a list or inspect its type. If the GraphQL type is nullable, the value is accepted; otherwise gqlparser returns the normal cannot be null validation error.

I have:

• Added tests covering the bug / feature
• Updated any relevant documentation

[coveralls]

coverage: 87.341% (-0.03%) from 87.371% — jbellenger:codex/fix-null-nested-list-coercion into vektah:master

[StevenACoffman]

Oh! Thank you so much! I can't tell you how much I appreciate you circling back here and not only reporting issues, not only fixing issues, but ADDING TESTS to prevent regressions.

I would give you a hug, but my arms are not long enough to reach from Ann Arbor, MI to Oakland, CA.