[bugfix,duneproject] Correctly quote maintainers to allow spaces.

Fixes flyspray issue 1418, see https://dune-project.org/flyspray/index.php?do=details&task_id=1418


(cherry picked from commit aae35424)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>

bin/duneproject

@@ -219,7 +219,7 @@ while [ "$DATACORRECT" != "y" -a "$DATACORRECT" != "Y" ]; do
   while [ -z $VERSION ]; do
     read -p "3) Project/Module version? " VERSION
   done
-  while [ -z $MAINTAINER ]; do
+  while [ -z "$MAINTAINER" ]; do
     read -p "4) Maintainers email address? " MAINTAINER
   done