Emit warning when emit_empty_windows is used unguarded.

[mfelsche]

And also rework the warnings api for the parser a bit.

This was an oversight from #828

Pull request

Description

We now create a warning like this when only emit_empty_windows is specified without guards like max_groups or eviction_period:

Warning: 
    1 | define tumbling window my_window
    2 | with
    3 |   interval = core::datetime::with_seconds(2),
    4 |   emit_empty_windows = true
    5 | end;
      | ^^^ Using `emit_empty_windows` without guard is potentially dangerous. Consider limiting the amount of groups maintained internally by using `max_groups` and/or `eviction_period`

Related

Checklist

• The RFC, if required, has been submitted and approved
• Any user-facing impact of the changes is reflected in docs.tremor.rs
• The code is tested
• Use of unsafe code is reasoned about in a comment
• Update CHANGELOG.md appropriately, recording any changes, bug fixes, or other observable changes in behavior
• The performance impact of the change is measured (see below)

Performance

[coveralls]

Coverage increased (+0.04%) to 81.702% when pulling 89286b6 on window-empty-warning into 18d4e3a on main.

[darach]

LGTM

[Licenser]

👍 Nice! I love the const'ed strings!