CWL Conditionals, when component doesn't handle condition when tool input parameter is a non-optional file with loadContents set to true

## Expected Behavior
I have a small workflow below that runs an expression step that collects the file contents.

I want to only run this step if the input is not null.

Although the input file is a mandatory input for the step, I would hope that my workflow could continue because I am skipping the step with the when condition.

## Actual Behavior

Unfortunately, because the tool input has a loadContents on the input file parameter, this is run before my 'when' condition on the step. Because the input file does not exist, this fails with the traceback below.

## Workflow Code
```cwl
class: Workflow
cwlVersion: v1.2

requirements:
InlineJavascriptRequirement: {}

inputs:
optional_small_file:
label: Optional File
type: File?
doc: |
This file is an optional file

steps:
get_file_contents:
in:
input_file:
source: optional_small_file
out:
- id: file_contents
when: >-
$( inputs.input_file !== null )
run:
class: ExpressionTool
cwlVersion: v1.2
inputs:
input_file:
type: File
loadContents: true
outputs:
file_contents:
type: string
expression: |
${
return {
"file_contents": inputs.input_file.contents
};
}

outputs:
optional_file_contents:
label: Optional file contents
type: string?
outputSource: get_file_contents/file_contents
```

## Full Traceback
```pytb
INFO /home/alexiswl/mambaforge/envs/cwl-ica/bin/cwltool 3.1.20240112164112
INFO Resolved 'load_contents_error_on_no_file.cwl' to 'file:///home/alexiswl/Projects/Personal/202401/cwltool-load-contents-issue/load_contents_error_on_no_file.cwl'
WARNING Workflow checker warning:
load_contents_error_on_no_file.cwl:10:5: Source 'optional_small_file' of type ["null", "File"] may
be incompatible
load_contents_error_on_no_file.cwl:17:7: with sink 'input_file' of type "File"
DEBUG Parsed job order from command line: {
"__id": "load_contents_error_on_no_file.cwl",
"optional_small_file": null
}
DEBUG [workflow ] initialized from file:///home/alexiswl/Projects/Personal/202401/cwltool-load-contents-issue/load_contents_error_on_no_file.cwl
INFO [workflow ] start
DEBUG [workflow ] inputs {
"optional_small_file": null
}
INFO [workflow ] starting step get_file_contents
DEBUG [step get_file_contents] job input {
"file:///home/alexiswl/Projects/Personal/202401/cwltool-load-contents-issue/load_contents_error_on_no_file.cwl#get_file_contents/input_file": null
}
ERROR Unhandled exception
Traceback (most recent call last):
File "/home/alexiswl/mambaforge/envs/cwl-ica/lib/python3.12/site-packages/cwltool/workflow_job.py", line 729, in try_make_job
inputobj = postScatterEval(inputobj)
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/alexiswl/mambaforge/envs/cwl-ica/lib/python3.12/site-packages/cwltool/workflow_job.py", line 637, in postScatterEval
if val.get("contents") is None:
^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get'
INFO [workflow ] completed permanentFail
DEBUG [workflow ] outputs {
"optional_file_contents": null
}
DEBUG Removing intermediate output directory /tmp/3zy6qnm7
{
"optional_file_contents": null
}WARNING Final process status is permanentFail
```

## Your Environment
* cwltool version: 3.1.20240112164112
Check using ``cwltool --version``