Exclude empty files in upload

libcloud seems to have issues when trying to upload empty files. although it's a libcloud issue, empty files could easily be removed in the  push_summary.

> libcloud.common.types.LibcloudError: <LibcloudError in <libcloud.storage.drivers.azure_blobs.AzureBlobsStorageDriver object at 0x7fb5e4f1d890> 'Unexpected status code, status_code=403'>