module: refactor condition

PR-URL: https://github.com/nodejs/node/pull/32989
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>

lib/internal/errors.js

@@ -1117,8 +1117,7 @@ E('ERR_INVALID_PACKAGE_TARGET',
       return `Invalid "exports" main target ${JSONStringify(target)} defined ` +
       `in the package config ${pkgPath}${sep}package.json${relError ?
         '; targets must start with "./"' : ''}`;
-    } else if (typeof target === 'string' && target !== '' &&
-      !StringPrototypeStartsWith(target, './')) {
+    } else if (relError) {
       return `Invalid "exports" target ${JSONStringify(target)} defined for '${
         StringPrototypeSlice(key, 0, -subpath.length || key.length)}' in the ` +
         `package config ${pkgPath}${sep}package.json; ` +