Add pipeline and subworkflow linting for ifEmpty(null)

Resolves issue #2506

Description from original issue

There are two general cases for workflows to use the channel operator ifEmpty. The first is ifEmpty( [ ] ) to ensure a process executes, for example when an input file is optional (although this can be replaced by toList()). The second is when a channel should not be empty and throws an error ifEmpty { error ... }, e.g. reading from an empty samplesheet.

There are multiple examples of workflows that inject null objects into channels using ifEmpty (see https://github.com/search?q=org%3Anf-core+ifEmpty&type=code&p=2), which could cause unhandled null pointer exceptions as was the case in #1966.
The feature request is that linting throws up a warning when this operation is found.

Warning message example:

Implementation based on pipeline_todos as it's very similar (link to main file)

Uses regex pattern ifEmpty\(\s*null\s*\)

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated