Fix NPE in IdenticalExpressionsDiagnostic when ctx.children is null

[Copilot]

• Understand the issue: NullPointerException occurs when ctx.children is null in sufficientSize method
• Fix using getChildCount() method instead of direct field access (per reviewer suggestion)
• Run existing tests to validate the fix (all 4 tests passed)
• Run code review
• Run codeql checker (no security alerts)

Original prompt

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This section details on the original issue you should resolve

<issue_title>NullPointerException: Cannot invoke "java.util.List.size()" because "ctx.children" is null</issue_title>
<issue_description>Sentry Issue: BSL-LANGUAGE-SERVER-MP

NullPointerException: Cannot invoke "java.util.List.size()" because "ctx.children" is null
    at com.github._1c_syntax.bsl.languageserver.diagnostics.IdenticalExpressionsDiagnostic.sufficientSize(IdenticalExpressionsDiagnostic.java:238)
    at com.github._1c_syntax.bsl.languageserver.diagnostics.IdenticalExpressionsDiagnostic.onExpressionEnter(IdenticalExpressionsDiagnostic.java:99)
    at com.github._1c_syntax.bsl.languageserver.diagnostics.AbstractExpressionTreeDiagnostic$ExpressionTreeBuilder.visitExpression(AbstractExpressionTreeDiagnostic.java:101)
    at com.github._1c_syntax.bsl.languageserver.diagnostics.AbstractExpressionTreeDiagnostic$ExpressionTreeBuilder.visitExpression(AbstractExpressionTreeDiagnostic.java:94)
    at com.github._1c_syntax.bsl.parser.BSLParser$ExpressionContext.accept(BSLParser.java:6697)
...
(56 additional frame(s) were not displayed)
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes NullPointerException: Cannot invoke "java.util.List.size()" because "ctx.children" is null #3678

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[nixel2007]

@copilot держи грамматику https://github.com/1c-syntax/bsl-parser/blob/develop/src/main/antlr/BSLParser.g4

[github-actions]

Test Results

2 700 files  ±0  2 700 suites  ±0   49m 11s ⏱️ +39s
1 013 tests ±0  1 013 ✅ ±0  0 💤 ±0  0 ❌ ±0 
9 117 runs  ±0  9 117 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 0db3439. ± Comparison against base commit 963574c.

♻️ This comment has been updated with latest results.

[Copilot]

Pull request overview

This PR fixes a NullPointerException in IdenticalExpressionsDiagnostic that occurs when ctx.children is null. The fix replaces direct field access (ctx.children.size()) with the ANTLR-provided method (ctx.getChildCount()), which handles null cases properly.

Key Changes

• Modified the sufficientSize() method to use getChildCount() instead of direct field access
• The change prevents NPE when processing expressions with null children