fix(core): enhance empty string expression validation error context (#8762)

[subbareddypalagiri]

This PR adds a protective context message to the ExpressionValidator to help users debug empty string issues efficiently, addressing the spam mentioned in Issue #8762.

[4ian]

An expression is not necessarily an event parameter, it can be used also in a event for example as the number of repeat for a repeat event or a limit in a for each.
So I think updating this error message is not the right solution :)

[subbareddypalagiri]

Thank you for the valuable feedback! I understand now that expressions can be used in various contexts within an event (e.g., repeat counts, limits) and are not exclusively instruction parameters. My previous change to make the error message more specific to 'parameters' was indeed incorrect and narrowed the context too much.

I've reverted the change to this error message to keep it general, ensuring it's applicable across all expression usages. I'll be more mindful of the broader context of expressions in future validations.