[no-unnecessary-type-assertion] False positive with null/undefined union type

[codebutler]

Looks like the fix for #529 did not cover all edge cases.

{
  "rules": {
      "@typescript-eslint/no-unnecessary-type-assertion": "error",
  }
}

const optionalNonNull = (s?: string) => { };
let s: string | null | undefined;

/*
Argument of type 'string | null | undefined' is not assignable to parameter of type 'string | undefined'.
  Type 'null' is not assignable to type 'string | undefined'.
*/
optionalNonNull(s);

/*
This assertion is unnecessary since the receiver accepts the original type of the expression.
*/
optionalNonNull(s!);

Expected Result

Should not trigger an error.

Actual Result

Triggers an error.

Additional Info

N/A

Versions

"@typescript-eslint/eslint-plugin": "^2.3.3",

Also tested with master 054df27

[bradzacher]

#1002 fixes this