roads-thematic-accuracy-indicator: return value 0 if no matched roads

I saw that if no matched roads are detected, then the value is NULL. Would maybe make more sense to set the value to 0 so that is comparable to "real NULL values.

No files or tests are named in the issue. Start by locating the roads-thematic-accuracy-indicator implementation and its existing tests, then trace the no-matched-roads case. Done means that case returns 0 while genuine NULL values remain distinguishable.

Written by the indexing model from the issue text.