Show no preview when the address does not reach Vim

Ex mode starts on the last line, so an empty $FZFVIM_EXCMD centered the preview
at the end of the file and said nothing. Quit with an error instead, which the
existing '|| exit' turns into a blank preview.

bin/tagpreview.sh

@@ -38,8 +38,15 @@ fi
 export FZFVIM_EXCMD="${EXCMD}"
 # WSL shares only the variables named in WSLENV with a Windows vim.exe
 export WSLENV="FZFVIM_EXCMD/w${WSLENV:+:${WSLENV}}"
+# Ex mode starts on the last line, so an address that did not reach Vim would
+# center the preview there rather than say anything. Show nothing instead
+REQUIRE_EXCMD=""
+if [ -n "${EXCMD}" ]; then
+  REQUIRE_EXCMD='if empty($FZFVIM_EXCMD) | cquit | endif'
+fi
 CENTER="$("${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
               -c "set nomagic" \
+              -c "${REQUIRE_EXCMD}" \
               -c 'silent sandbox execute $FZFVIM_EXCMD' \
               -c 'let l=line(".") | new | put =l | print | qa!')" || exit