Polish assertions

See gh-23762

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jdbc/DataSourcePropertiesTests.java

@@ -103,6 +103,7 @@ class DataSourcePropertiesTests {
 		DataSourceProperties properties = new DataSourceProperties();
 		properties.setUsername("");
 		properties.afterPropertiesSet();
+		assertThat(properties.getUsername()).isEqualTo("");
 		assertThat(properties.determineUsername()).isEqualTo("sa");
 	}
 
@@ -111,6 +112,7 @@ class DataSourcePropertiesTests {
 		DataSourceProperties properties = new DataSourceProperties();
 		properties.setUsername(null);
 		properties.afterPropertiesSet();
+		assertThat(properties.getUsername()).isNull();
 		assertThat(properties.determineUsername()).isEqualTo("sa");
 	}