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/main/java/org/springframework/boot/autoconfigure/flyway/FlywayProperties.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2022 the original author or authors.
+ * Copyright 2012-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -601,7 +601,7 @@ public class FlywayProperties {
 	}
 
 	public String getPassword() {
-		return (this.password != null) ? this.password : "";
+		return this.password;
 	}
 
 	public void setPassword(String password) {

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);
 			});
 	}