Allow empty log lines to be sent from the worker

[stuartc]

Refactor log message handling to allow nil values and improve error validation.

console.log();          => new line
console.log('');        => new line
console.log(null);      => null
console.log(undefined); => new line

Closes #2914

Description

We now allow the worker to send through empty log lines.

"Empty" in this case means 'something', the default value for the schema is now nil, and so "" is valid because thats the value from the worker.

We still reject some messages, if the worker doesn't send anything for a LogLine message, that's still invalid.

Validation steps

This is not 1:1 demonstrable in a dev environment, because the error in Sentry is a "soft-error". There is not exception as such but rather a "error" logged to Sentry.

We never really tested for blank lines because (well 😅) it ignored them.

Applying this diff to main:

diff --git a/test/integration/web_and_worker_test.exs b/test/integration/web_and_worker_test.exs
index 263d1c459..f3c843d2d 100644
--- a/test/integration/web_and_worker_test.exs
+++ b/test/integration/web_and_worker_test.exs
@@ -346,6 +353,10 @@ defmodule Lightning.WebAndWorkerTest do
 
   defp flow_expression do
     "fn(state => {
+      console.log();
+      console.log('');
+      console.log(null);
+      console.log(undefined);
       console.log(state.configuration);
       throw 'fail!'
     });"

And adding something like a raise "hell" to reply_with/2 at lib/lightning_web/channels/channel_helpers.ex:7 and then run mix test test/integration/web_and_worker_test.exs; will result in a test failure.

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Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
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[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.52%. Comparing base (70112a0) to head (4572671).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/lightning/invocation/log_line.ex	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2915      +/-   ##
==========================================
+ Coverage   91.49%   91.52%   +0.02%     
==========================================
  Files         344      344              
  Lines       12509    12512       +3     
==========================================
+ Hits        11445    11451       +6     
+ Misses       1064     1061       -3     

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