### Motivation
Right now, while creating subscription if user doesn't pass message-id then
broker doesn't handle it properly which gives NPE and gives internal-server
error to user.
```
Message: null
Stacktrace:
java.lang.NullPointerException
at
org.apache.pulsar.broker.admin.PersistentTopics.createSubscription(PersistentTopics.java:885)
at sun.reflect.GeneratedMethodAccessor199.invoke(Unknown Source)
```
### Modifications
- take earliest-message id while creating subscription if user doesn't provide
message-id.
[ Full content available at: https://github.com/apache/pulsar/pull/2636 ]
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