🐛 : NPE with ldap healthcheck

[juwit]

Describe the bug
When gaia is connected to a LDAP server, spring-boot-actuator health performs LDAP healthcheck.

Workaround
Set the property management.health.ldap.enabled=false.
Wait to spring-ldap 2.3.3 to be released.

To Reproduce
Steps to reproduce the behavior:

1. Connect Gaia to a LDAP server
2. Go to {url}/admin/health
3. See 503 error with DOWN status
4. See exception in stacktrace

Expected behavior
Status should be UP with no exception

Stacktrace

2019-08-19 08:08:34.547 TRACE 1 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : GET "/admin/health", parameters={}, headers={masked} in DispatcherServlet 'dispatcherServlet'
2019-08-19 08:08:34.548 TRACE 1 --- [nio-8080-exec-1] o.s.w.m.support.InvocableHandlerMethod   : Arguments: [null]
2019-08-19 08:08:34.548 TRACE 1 --- [nio-8080-exec-1] o.s.w.m.support.InvocableHandlerMethod   : Arguments: [null, null]
2019-08-19 08:08:34.548 TRACE 1 --- [nio-8080-exec-1] .w.s.m.m.a.ServletInvocableHandlerMethod : Arguments: [SecurityContextHolderAwareRequestWrapper[ org.springframework.security.web.header.HeaderWriterFilter$HeaderWriterRequest@13782a45], null]
2019-08-19 08:08:34.550  WARN 1 --- [nio-8080-exec-1] o.s.b.actuate.ldap.LdapHealthIndicator   : LDAP health check failed

java.lang.NullPointerException: null
	at java.base/java.util.Hashtable.put(Hashtable.java:475) ~[na:na]
	at org.springframework.ldap.core.support.SimpleDirContextAuthenticationStrategy.setupEnvironment(SimpleDirContextAuthenticationStrategy.java:42) ~[spring-ldap-core-2.3.2.RELEASE.jar!/:2.3.2.RELEASE]
	at org.springframework.ldap.core.support.AbstractContextSource.setupAuthenticatedEnvironment(AbstractContextSource.java:194) ~[spring-ldap-core-2.3.2.RELEASE.jar!/:2.3.2.RELEASE]
	at org.springframework.ldap.core.support.AbstractContextSource.getAuthenticatedEnv(AbstractContextSource.java:582) ~[spring-ldap-core-2.3.2.RELEASE.jar!/:2.3.2.RELEASE]
	at org.springframework.ldap.core.support.AbstractContextSource.doGetContext(AbstractContextSource.java:134) ~[spring-ldap-core-2.3.2.RELEASE.jar!/:2.3.2.RELEASE]
	at org.springframework.ldap.core.support.AbstractContextSource.getReadOnlyContext(AbstractContextSource.java:158) ~[spring-ldap-core-2.3.2.RELEASE.jar!/:2.3.2.RELEASE]
	at org.springframework.ldap.core.LdapTemplate.executeReadOnly(LdapTemplate.java:802) ~[spring-ldap-core-2.3.2.RELEASE.jar

Desktop (please complete the following information):

• OS: Centos 7.6
• Browser : firefox
• Version: 68.0.2

Additional context
Related to a spring-ldap issue : spring-projects/spring-ldap#473.