sysinstall: don't ask whether to erase a disk that contains no partitions

When the target disk carried no partitions, the installer still asked:

    Please type 'keep' to install the OS in addition to what the disk
    already contains, or 'erase' to erase all data on the disk:

The question is meaningless in that case: there's nothing on the disk
worth preserving, and both answers lead to the same result.

Modify fsystemd-repart to report in the dry-run reply of
io.systemd.Repart.Run() the number of partitions currently on the disk.
The field is only included if the existing partition table was actually
read, i.e. in the 'refuse' and 'allow' empty modes, and omitted
otherwise.

Make systemd-sysinstall skip the erase question if the reply positively
indicates that there are no partitions, proceeding as if 'keep' was
selected.

src/shared/varlink-io.systemd.Repart.c

@@ -59,6 +59,8 @@ static SD_VARLINK_DEFINE_METHOD_FULL(
                 SD_VARLINK_DEFINE_OUTPUT(minimalSizeBytes, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("In dry-run mode returns the size of the selected block device."),
                 SD_VARLINK_DEFINE_OUTPUT(currentSizeBytes, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Returns the number of partitions currently on the selected block device. Omitted if not determined by the workflow, e.g. if the existing partition table was not read because empty mode 'force' or 'require' was selected."),
+                SD_VARLINK_DEFINE_OUTPUT(partitionCount, SD_VARLINK_INT, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("If used with the 'more' flag, a phase identifier is sent in progress updates."),
                 SD_VARLINK_DEFINE_OUTPUT_BY_TYPE(phase, ProgressPhase, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("If used with the 'more' flag, an object identifier string is sent in progress updates."),