There is no way to tell CustomDimension to omit NULL values in reports. All entries with NULL values are reported as
Value not defined in report table.
Scenario when I want to have only specific visits reported in in CustomDimension report is not possible.
Moved to CustomDimensions repository: https://github.com/piwik/plugin-CustomDimensions/issues/32
This should be closed.
Since the CustomDimensions repository linked above has vanished and the new one does not have any issues, what's the answer to the original issue? Behaviour is still as described there...