Replace truthy check with isset when setting default checkout field value (#48031)

Co-authored-by: Thomas Roberts <5656702+opr@users.noreply.github.com>

plugins/woocommerce/changelog/pr-48031

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Fix a bug with the woocommerce_get_default_value_for_{key} filter that was preventing setting a falsey value on a checkbox (i.e. to uncheck it dynamically)

plugins/woocommerce/src/Blocks/Domain/Services/CheckoutFields.php

@@ -1187,7 +1187,7 @@ class CheckoutFields {
 				 */
 				$value = apply_filters( "woocommerce_get_default_value_for_{$missing_field}", null, $group, $wc_object );
 
-			if ( $value ) {
+			if ( isset( $value ) ) {
 				$meta_data[ $missing_field ] = $value;
 			}
 		}