Issue #505: Default env var to an empty string

KOHA_CI_INCREMENTAL_RUN_REPO_URL: ${KOHA_CI_INCREMENTAL_RUN_REPO_URL:-""}

This is wrong, we should default to an... empty string, not two quotes.

% perl -MModern::Perl -e 'my $url = $ENV{KOHA_CI_INCREMENTAL_RUN_REPO_URL}; say $url;' ""

And so:

perl -MModern::Perl -e 'my $url = $ENV{KOHA_CI_INCREMENTAL_RUN_REPO_URL} // "THE url"; say $url;' ""

Which breaks the default in Koha/Devel/CI/IncrementalRuns.pm