Fix: Prevent null reference error in drag-and-drop sorting

[rainerregan]

Problem:

• Custom drag-and-drop operations were failing with DropLocationDeterminer.ts:254 Uncaught TypeError: Cannot read properties of undefined (reading 'clone')
• Occurred when component children existed in the model but didn't have valid DOM dimensions for sorting
  Mismatch between [children.length > 0]
• Mismatch between children.length > 0 and empty childrenDimensions array caused undefined placeholderDimensions

Solution:

• Adding condition to check nodeHasChildren and childrenDimensions.length > 0 before processing the positioning.
• Adding null check on nodeDimensions.clone() to make sure placeholderDimensions is defined.
• Prevent crashes when creating custom logic for drag and drop operation.

[artf]

Hey thanks @rainerregan, looks like a legit fix but I'm wondering if by any chance you have an HTML snippet (the model but didn't have valid DOM dimensions for sorting) that could reproduce the issue