Add a default back to INFOPATH

It looks like #7738 removed the default unintentionally. This adds it back. 

For scripts that `set -u`, this will fail if $INFOPATH is not already set (`INFOPATH: unbound variable`). This provides an empty default, but one that will still satisfy the trailing colon requirement.

Library/Homebrew/cmd/shellenv.sh

@@ -29,7 +29,7 @@ homebrew-shellenv() {
       echo "export HOMEBREW_REPOSITORY=\"$HOMEBREW_REPOSITORY\";"
       echo "export PATH=\"$HOMEBREW_PREFIX/bin:$HOMEBREW_PREFIX/sbin\${PATH+:\$PATH}\";"
       echo "export MANPATH=\"$HOMEBREW_PREFIX/share/man\${MANPATH+:\$MANPATH}:\";"
-      echo "export INFOPATH=\"$HOMEBREW_PREFIX/share/info:\${INFOPATH}\";"
+      echo "export INFOPATH=\"$HOMEBREW_PREFIX/share/info:\${INFOPATH:-}\";"
       ;;
   esac
 }