Hi Neil,
I received some feedback about my reply… and I stand corrected. Here’s the feedback which should be helpful to you:
The WP_Query docs feature heaps of ‘meta’ args examples including ’empty’ – https://codex.wordpress.org/Class_Reference/WP_Query
I believe the meta_query value can be set to ‘NOT EXISTS’.
Also, a repeater field with 0 rows will be saved with a value of ‘0’, not ‘false’.
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My apologies for incorrect info in my original reply.