Do not turn a null Flyway-specific password into an empty string

It prevents using PGPASS for authentication with Postgres.

Fixes gh-35110

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/flyway/FlywayAutoConfigurationTests.java

@@ -154,7 +154,7 @@ class FlywayAutoConfigurationTests {
 				DataSource dataSource = context.getBean(Flyway.class).getConfiguration().getDataSource();
 				assertThat(dataSource).isNotNull();
 				assertThat(dataSource).hasFieldOrPropertyWithValue("username", null);
-				assertThat(dataSource).hasFieldOrPropertyWithValue("password", "");
+				assertThat(dataSource).hasFieldOrPropertyWithValue("password", null);
 			});
 	}