Extend 'file exists' check to require non-zero size

Somehow it is possible to get into a state where the privkey.pem file
has a zero size when using LOCAL_CA. How this is achieved is not
confirmed, but I suspect that if openssl is interrupted during
its process it just truncates the file.

With a simple change from -f to -s we now also require that all of
the important files has a size greater than 0. The caveat with this
is that a file containing a single newline character will be accepted,
but let's start here and see if we need even more advanced checks.

Resolves #185

src/scripts/util.sh

@@ -156,8 +156,8 @@ allfiles_exist() {
                 debug "Ignoring ${type} path starting with 'data:' in '${1}'"
             elif [[ "${path}" == engine:* ]]; then
                 debug "Ignoring ${type} path starting with 'engine:' in '${1}'"
-            elif [ ! -f "${path}" ]; then
-                warning "Could not find ${type} file '${path}' in '${1}'"
+            elif [ ! -s "${path}" ]; then
+                warning "Could not find non-zero size ${type} file '${path}' in '${1}'"
                 all_exist=1
             fi
         done
@@ -298,7 +298,7 @@ auto_enable_configs() {
             fi
         else
             if [ "${conf_file##*.}" = "conf" ]; then
-                error "Important file(s) for '${conf_file}' are missing, disabling..."
+                error "Important file(s) for '${conf_file}' are missing or empty, disabling..."
                 mv "${conf_file}" "${conf_file}.nokey"
             fi
         fi