fix: include null in return type

Author: Planeshifter

lib/node_modules/@stdlib/os/configdir/docs/types/index.d.ts

@@ -37,7 +37,7 @@
 * var dir = configdir( 'appname/config' );
 * // e.g., returns '/Users/<username>/Library/Preferences/appname/config'
 */
-declare function configdir( p?: string ): string;
+declare function configdir( p?: string ): string | null;
 
 
 // EXPORTS //

lib/node_modules/@stdlib/os/configdir/docs/types/test.ts

@@ -23,8 +23,8 @@ import configdir = require( './index' );
 
 // The function returns a string...
 {
-	configdir(); // $ExpectType string
-	configdir( 'appname/config' ); // $ExpectType string
+	configdir(); // $ExpectType string | null
+	configdir( 'appname/config' ); // $ExpectType string | null
 }
 
 // The compiler throws an error if the function is provided a value other than a string...