Error-Handling of empty NAS-files

[frankjaeger]

While updating an area with NAS (ALKIS) some tiles contains no objekt to update. The NAS-File has no objekt and no geometry, but contains XML-frame-stuff and is about 3000 byte in size.

This files can be ignored or responded by a warning "empty". But the error-Message is:

FAILURE:
Unable to open datasource `path/file.xml' with the following drivers.
... followed by a long list of drivers, growing with each release.

By checking the error-list it's a lot of lines to scroll because nearly each second file is empty.
Please make a short message for empty NAS-tiles.

[frankjaeger]

NAS-XML-file with no Objekts, emty tile

[warmerdam]

In (r22088) I have modified the NAS driver to allow opening empty files without an error. Added testing too (r22089).