feat: make msls_get_switcher() $attr optional

Give $attr a default of array() so the documented template tag
can be called with no arguments. The body already coerced any
non-array value to array(), so $attr was effectively optional;
the missing default made msls_get_switcher() throw an
ArgumentCountError on the natural no-arg call.

The parameter stays untyped to preserve the is_array() coercion,
which keeps the [sc_msls] shortcode working (WordPress passes ''
when a shortcode has no attributes).

Closes #643

Co-Authored-By: Claude <noreply@anthropic.com>

Author: apermo

includes/api.php

@@ -10,10 +10,11 @@
  * Get the output for using the links to the translations in your code
  *
  * @package Msls
- * @param mixed $attr
+ * @since 3.0.0 The $attr parameter is optional and defaults to an empty array.
+ * @param mixed $attr Optional. Attributes forwarded to the switcher output. Default empty array.
  * @return string
  */
-function msls_get_switcher( $attr ): string {
+function msls_get_switcher( $attr = array() ): string {
 	$arr = is_array( $attr ) ? $attr : array();
 	$obj = apply_filters( 'msls_get_output', null );