examples: avoid "unbound variable" errors

When the shell option `nounset` is set, we may get "unbound variable"
errors.
https://lists.gnu.org/r/bug-bison/2020-11/msg00013.html

* examples/test (diff_opts): Be sure to initialize it.

examples/test

@@ -37,6 +37,7 @@ strip_prompt=false
 # If diff supports --strip-trailing-cr, use it, to avoid EOL issues
 # when testing Java programs on Windows.
 echo "checking for diff --strip-trailing-cr..."
+diff_opts=
 if diff --strip-trailing-cr "$1" "$1"; then
   diff_opts=--strip-trailing-cr
 fi