ProviderManager should have a varargs constructor

- Added varargs constructor to ProviderManager.
- Added check for null values in AuthenticationProvider list.
- Updated ProviderManagerTests to test for null values using both constructors.

Fixes gh-7713

core/src/test/java/org/springframework/security/authentication/ProviderManagerTests.java

@@ -95,8 +95,13 @@ public class ProviderManagerTests {
 	}
 
 	@Test(expected = IllegalArgumentException.class)
-	public void testStartupFailsIfProvidersNotSet() {
-		new ProviderManager(null);
+	public void testStartupFailsIfProvidersNotSetAsList() {
+		new ProviderManager((List<AuthenticationProvider>) null);
+	}
+
+	@Test(expected = IllegalArgumentException.class)
+	public void testStartupFailsIfProvidersNotSetAsVarargs() {
+		new ProviderManager((AuthenticationProvider) null);
 	}
 
 	@Test