test: add case for mean function to return NaN when inputs are zero

Author: sethdivyansh

lib/node_modules/@stdlib/stats/base/dists/cosine/mean/test/test.native.js

@@ -70,6 +70,9 @@ tape( 'if provided `s <= 0.0`, the function returns `NaN`', opts, function test(
 	y = mean( 2.0, -2.0 );
 	t.equal( isnan( y ), true, 'returns NaN' );
 
+	y = mean( 0.0, 0.0 );
+	t.equal( isnan( y ), true, 'returns NaN' );
+
 	y = mean( NINF, NINF );
 	t.equal( isnan( y ), true, 'returns NaN' );