Fix for issue #1108

[InAnYan]

Issue

Solves #1108.

Change

• The default value of id in MessageWindowChatMemory.Builder is null.
• The constructor of MessageWindowChatMemory checks for null id value. If it is null, then it will generate a random UUID.
• Updated the documentation for MessageWindowChatMemory.Builder.id().

General checklist

• There are no breaking changes
• I have added unit and integration tests for my change
• I have manually run all the unit and integration tests in the module I have added/changed, and they are all green
• I have manually run all the unit and integration tests in the core and main modules, and they are all green

• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)

[langchain4j]

Closing it as #1108 was closed