repo-add: unconditionally create the database if it is missing

In f91fa54 (repo-add: do not recreate the database if nothing was changed),
repo-add was made to skip database write-out if there were no changes to
the database. However, this breaks the usage of repo-add to create a new
empty database: `repo-add /path/to/mydb/mydb.db.tar.xz`.

Bring back support for this use-case by always writing the database if
it is missing.

Fixes: f91fa54

scripts/repo-add.sh.in

@@ -44,6 +44,7 @@ USE_COLOR='y'
 PREVENT_DOWNGRADE=0
 INCLUDE_SIGS=0
 DB_MODIFIED=0
+DB_MISSING=0
 
 # Import libmakepkg
 source "$MAKEPKG_LIBRARY"/util/compress.sh
@@ -440,6 +441,8 @@ prepare_repo_db() {
 						exit 1
 					fi
 					rm -f "$dbfile"
+					# mark the db for unconditional update
+					DB_MISSING=1
 					;;
 			esac
 		fi
@@ -710,7 +713,7 @@ if (( fail )); then
 	exit 1
 fi
 
-if (( DB_MODIFIED )); then
+if (( DB_MODIFIED || DB_MISSING )); then
 	msg "$(gettext "Creating updated database file '%s'")" "$REPO_DB_FILE"
 	create_db
 	rotate_db