Allow equals with ignoreCase parameter to take nullable Strings both as a receiver and a parameter.

#KT-9188

libraries/stdlib/test/text/StringTest.kt

@@ -528,6 +528,10 @@ class StringTest {
     @test fun equalsIgnoreCase() {
         assertFalse("sample".equals("Sample", ignoreCase = false))
         assertTrue("sample".equals("Sample", ignoreCase = true))
+        assertFalse("sample".equals(null, ignoreCase = false))
+        assertFalse("sample".equals(null, ignoreCase = true))
+        assertTrue(null.equals(null, ignoreCase = true))
+        assertTrue(null.equals(null, ignoreCase = false))
     }