util: make issue must save empty variables

.tcl scripts check for the presence of variables sometimes. Hence
an empty enviornment variable is not the same as an absent environment
variable.

Therefore empty environment variables have to be saved, in general.

This reverts parts of commit 0a4b13f.

Signed-off-by: Øyvind Harboe <[email protected]>

Author: oharboe

flow/util/generate-vars.sh

@@ -20,10 +20,6 @@ do
         continue
     fi
     rhs=`sed -e 's/^"//' -e 's/"$//' <<<"${V#*=}"`
-    if [[ "${rhs}" == "''" ]]; then
-        echo "Skiping empty variable ${V}"
-        continue
-    fi
     # handle absolute paths
     if [[ "${rhs}" == /* ]]; then
         if [[ ! -e "${rhs}" ]]; then