Fix sandbox.tt variable comparison usage

When an uninitialized variable is given to `-n`, it is treated as not
NULL. The variable must be quoted for correct results.
`[ -n "$SOLIDUS_BRANCH" ]`

It is also recommended for variable comparisons in general as it can
produce incorrect results for `! -z`.

References:
- https://tldp.org/LDP/abs/html/comparison-ops.html
- https://tldp.org/LDP/abs/html/comparison-ops.html#STRTEST

This meant that before, when running this file with no SOLIDUS_BRANCH
variable, $BRANCH would end up NULL and the branch in the Gemfile would
be an empty string. This caused an error, whereas instead, it should
have defaulted to `master`.

Error:
```
fatal: Needed a single revision
Git error: command `git rev-parse --verify ''` in directory /Users/ryanwoods/.rbenv/versions/2.7.6/lib/ruby/gems/2.7.0/cache/bundler/git/solidus-169f1ecb1aee2122950e6d586daf2410f62df66e has
failed.
Revision  does not exist in the repository https://github.com/solidusio/solidus.git. Maybe you misspelled it?
If this error persists you could try removing the cache directory
'/Users/ryanwoods/.rbenv/versions/2.7.6/lib/ruby/gems/2.7.0/cache/bundler/git/solidus-169f1ecb1aee2122950e6d586daf2410f62df66e'

The git source https://github.com/solidusio/solidus.git is not yet checked out. Please run `bundle install` before trying to start your application
```

lib/solidus_dev_support/templates/extension/bin/sandbox.tt

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 
 set -e
-if [ ! -z $DEBUG ]
+if [ ! -z "$DEBUG" ]
 then
   set -x
 fi
@@ -28,7 +28,7 @@ sqlite3|sqlite)
 esac
 echo "~~> Using $RAILSDB as the database engine"
 
-if [ -n $SOLIDUS_BRANCH ]
+if [ -n "$SOLIDUS_BRANCH" ]
 then
   BRANCH=$SOLIDUS_BRANCH
 else
@@ -37,7 +37,7 @@ else
 fi
 echo "~~> Using branch $BRANCH of solidus"
 
-if [ -z $SOLIDUS_FRONTEND ]
+if [ -z "$SOLIDUS_FRONTEND" ]
 then
   echo "~~> Use 'export SOLIDUS_FRONTEND=[solidus_frontend|solidus_starter_frontend]' to control the Solidus frontend"
   SOLIDUS_FRONTEND="solidus_frontend"