ExpressionTool doesn't check output types

CWL Specification explicitly states that

> The `Any` type validates for any non-null value.

However, cwtool does not fail when a step produces a null output for a port of type `Any`. Instead, the behavior is correct for input ports. As a concrete example, the following `ExpressionTool` should clearly fail, but it doesn't.

```yaml
#!/usr/bin/env cwl-runner

class: ExpressionTool
requirements: { InlineJavascriptRequirement: {} }
cwlVersion: v1.0

inputs: []

outputs:
output: Any

expression: "$({'output': null })"
```

Note also that this file is involved in 2 CWL Conformance tests cases, `step_input_default_value_overriden_2nd_step_null` and `step_input_default_value_overriden_2nd_step_null_noexp`. Therefore, it is quite crucial to establish the correct behavior here.

Run the supplied cwl-runner ExpressionTool example first, then trace ExpressionTool output validation and the conformance cases step_input_default_value_overriden_2nd_step_null and step_input_default_value_overriden_2nd_step_null_noexp. Done means a null output for Any fails while input-port behavior remains correct and the conformance cases reflect the specification.

Written by the indexing model from the issue text.