Add check for empty label

[gvzdv]

Resolves #1143.

If any of the stage labels is empty, the preprocessor will return 204.
This prevents running most of the multistage-diagram-segmentation steps on non-diagram graphics and incomplete diagrams.

Tested on Unicorn with non-diagram graphics.

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[gvzdv]

Good for now. We should revisit to make sure we're not excluding valid diagrams if we're missing one or two labels.

Future enhancement: experiment with letting the model assign the label if it's missing in the diagram (after we update categorization).