fix(markdown-common): retain empty nodes arrays in AST to satisfy strict=true in concerto-v4

[rockaxorb13]

Description

fixes #651
This PR fixes a critical AST validation crash that occurs when using Concerto v4 engine (specifically 4.0.0-beta.1 and later) alongside markdown-transform. I uncovered this bug 2-3 weeks ago and and the fix was easier than I anticipated!

The Root Cause:
As described in the issue:
In packages/markdown-common/lib/FromMarkdownIt.js (blockToCommonMark), the parser was explicitly deleting the nodes array if a markdown element (like an empty clause, list, or blockquote) had no children. While Concerto v3 permitted this "lazy AST" where the array was simply undefined, the new strict type-checking in Concerto v4 throws a fatal ValidationException: Expected property 'nodes' to be an array because nodes is defined as a mandatory array in the metamodel.

The Fix:
Removed the aggressive deletion block (delete node.nodes) in FromMarkdownIt.js so that empty elements correctly retain an empty array ("nodes": []).

Testing / Validation

• Tested against Concerto v3: Fully backward compatible. Ran the full test suite and updated Jest snapshots successfully.
• Tested against Concerto v4: Prevents the ValidationException crash in strict parsing environments.

Standalone Script Output Proof:
I made a script to directly test this:
By feeding an empty clause ({{#clause testClause}}\n{{/clause}}) into CiceroMarkTransformer, we can see the exact structural change to the AST:

Before (Fails v4 Validation):

--- RAW AST OUTPUT ---
{
  "$class": "org.accordproject.ciceromark@0.6.0.Clause",
  "name": "testClause"
}
❌ BUG REPRODUCED: 'nodes' array was deleted!

After (Passes v4 Validation):

--- RAW AST OUTPUT ---
{
  "$class": "org.accordproject.ciceromark@0.6.0.Clause",
  "name": "testClause",
  "nodes": []
}
✅ FIX WORKS: 'nodes' array is present!

[mttrbrts]

Great find!

[coveralls]

Coverage Report for CI Build 24067562976

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Coverage decreased (-0.08%) to 72.194%

Details

• Coverage decreased (-0.08%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

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Coverage Stats

Relevant Lines:	2613
Covered Lines:	1958
Line Coverage:	74.93%
Relevant Branches:	1156
Covered Branches:	763
Branch Coverage:	66.0%
Branches in Coverage %:	Yes
Coverage Strength:	1467.86 hits per line

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