Fix logging of asset count in impact calculation

[spjuhel]

In the following:

LOGGER.info(
            "Calculating impact for %s assets (>0) and %s events.",
            exp_gdf.size,
            self.n_events,
        )

gdf.size return the number of cells, not the number of rows, so the logging message is wrong.

This is easily fixed by switching to len(exp_gdf)

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[spjuhel]

@ValentinGebhart @emanuel-schmid

It should take 2min to review 😇

[spjuhel]

Oh wait I just saw that the tests checked the logging, will update ASAP.

[peanutfun]

Please fix the test and this is good to go! 😄