Fix ArrayIndexOutOfBoundsException in CompositeBitmapIndex null handling

[hrstoyanov]

Summary

Fixes ArrayIndexOutOfBoundsException in AbstractCompositeBitmapIndex.internalHandleChanged() when updating an entity from null to non-null composite key value (or vice versa).

Issue Description

When an entity with a composite-indexed field (e.g., IndexerInstant for Instant fields) is:

1. Added with a null value → composite key = NULL() = Object[1]
2. Updated to a non-null value → composite key = Object[6] (for Instant's 6 components)

The method would throw:

java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1
    at org.eclipse.store.gigamap.types.SubBitmapIndexHashing.index()

Root Cause

The original internalHandleChanged() method called ensureSubIndices(newKeys) which grew the subIndices array, then iterated all sub-indices calling internalHandleChanged(oldKeys, ...). When oldKeys was shorter than the new array (e.g., length 1 vs 6), accessing oldKeys[position] threw ArrayIndexOutOfBoundsException.

Solution

Modified internalHandleChanged() to check isEmpty(oldKeys, i) and isEmpty(newKeys, i) per sub-index position:

• Both empty: Skip (nothing to do)
• Old empty, new non-empty: ADD via internalAddToEntry()
• Old non-empty, new empty: REMOVE via internalRemove()
• Both non-empty: HANDLE CHANGE via internalHandleChanged()

Files Changed

1. AbstractCompositeBitmapIndex.java - Fixed internalHandleChanged() method (lines 529-568)
2. CompositeIndexNullHandlingTest.java (NEW) - Comprehensive test coverage with 5 test methods

Testing

Test covers:

• null→non-null updates
• non-null→null updates
• Persistence with EmbeddedStorage
• Multiple entity updates
• Query operations with mixed null/non-null values

Impact

• Breaking Changes: None
• Backward Compatible: Yes
• Performance Impact: Minimal (adds 2 boolean checks per iteration)

Discovered by TourBiz project during Booking entity updates with nullable Instant fields.

[hrstoyanov]

This addresses #623

[hrstoyanov]

I have 100+ tests for Eclipse Store and with this fix it all pass now.

Please release 4.0.2

[zdenek-jonas]

Thanks for the PR and the contribution. Due to our current review capacity, this will likely not be reviewed for at least a couple of weeks. Thank you for your patience.