When a Sensor saves empty coverage for a file, don't fallback on executable lines

If a coverage engine reports there are no lines to cover in a file, then we don't store any measure:
https://github.com/SonarSource/sonar-enterprise/blob/bf8b731205eab25d23f23384f751bcf38ad5d184/sonar-scanner-engine/src/main/java/org/sonar/scanner/sensor/DefaultSensorStorage.java#L490
And then, there is a fallback on using executable lines computed by the code analyzer:
https://github.com/SonarSource/sonar-enterprise/blob/master/sonar-scanner-engine/src/main/java/org/sonar/scanner/source/ZeroCoverageSensor.java#L66
That may be not what is expected by the user:
https://github.com/jacoco/jacoco/issues/806

We should "remember" that coverage was stored, even with no lines to cover, and only fallback on executable lines when the coverage was not saved on a file.