output: fix missing buffer when using direct scanout
When using direct scanout back_buffer is NULL. We'd emit a commit event with WLR_OUTPUT_STATE_BUFFER set but with a NULL buffer field, which is non-sensical.
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When using direct scanout back_buffer is NULL. We'd emit a commit event with WLR_OUTPUT_STATE_BUFFER set but with a NULL buffer field, which is non-sensical.