[no-unnecessary-type-assertion] false positive when asserting via non-null assertion operator

[vkrol]

Repro
https://github.com/vkrol/typescript-eslint-no-unnecessary-type-assertion-undefined

module.exports = {
	parser: "@typescript-eslint/parser",
	parserOptions: {
		project: "tsconfig.json",
	},
	plugins: ["@typescript-eslint"],
	rules: {
		"@typescript-eslint/no-unnecessary-type-assertion": "error",
	}
};

declare function foo(str?: string): void;
declare const str: string | null;

foo(str!);

Expected Result
No errors.
Actual Result
Error:

D:\Projects\typescript-eslint-no-unnecessary-type-assertion-undefined\index.ts
  4:5  error  This assertion is unnecessary since the receiver accepts the original type of the expression  @typescript-eslint/no-unnecessary-type-assertion

✖ 1 problem (1 error, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

Additional Info
The change that introduced this regression: #478.

Versions

package	version
@typescript-eslint/eslint-plugin	1.9.0
@typescript-eslint/parser	1.9.0
TypeScript	3.4.5
ESLint	5.16.0
node	10.15.3
npm	6.4.1

[vkrol]

@bradzacher I think that this change #478 introduced this regression.

[bradzacher]

no doubt - that PR added a lot of code and additional checks and had to work around uninitialised variables (#453).

this bug only occurs when strictNullChecks is turned on.