`LdapTemplate` constructor accepts a `null` `ContextSource`

Problem

The LadpTemplates constructor accepts null as a parameter.
https://github.com/spring-projects/spring-ldap/blob/1c75ae7d56d3a3cf8c58632d6ad5ecf578787954/core/src/main/java/org/springframework/ldap/core/LdapTemplate.java#L116-L122

The LdapTemlate#afterPropertySet overwrite check for the instance type of the context. null expected to be represented as NullContextSource. This will fail after a new LadpTemplate(null) invocation because null is not an instance of NullContextSource.
https://github.com/spring-projects/spring-ldap/blob/1c75ae7d56d3a3cf8c58632d6ad5ecf578787954/core/src/main/java/org/springframework/ldap/core/LdapTemplate.java#L1130-L1136

At the same time LdapTemplate#setContextSource check for nullness before setting by invoking Assert.notNull.
https://github.com/spring-projects/spring-ldap/blob/1c75ae7d56d3a3cf8c58632d6ad5ecf578787954/core/src/main/java/org/springframework/ldap/core/LdapTemplate.java#L124-L131

Expectation

Aligned semantics across the class regarding the nullness of contextSource.
So new LadpTemplate(null) should either:

• throw an exception
• instanciate LadpTemplate.contextSource an instance of NullContextSource

Start in core/src/main/java/org/springframework/ldap/core/LdapTemplate.java at the constructor, setContextSource, and afterPropertiesSet methods. Compare how each handles a null ContextSource, then add regression coverage showing that new LdapTemplate(null) follows the chosen consistent behavior: rejecting null or using NullContextSource.