Allow initiate_chat without passing message

[bitnom]

A very minor change to call self.get_human_input() if message is not supplied.

• [x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9729610) 33.34% compared to head (087a032) 66.53%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1244       +/-   ##
===========================================
+ Coverage   33.34%   66.53%   +33.19%     
===========================================
  Files          33       33               
  Lines        4456     4462        +6     
  Branches     1041     1098       +57     
===========================================
+ Hits         1486     2969     +1483     
+ Misses       2848     1182     -1666     
- Partials      122      311      +189     

Flag	Coverage Δ
unittests	66.49% <100.00%> (+33.19%)	:arrow_up:

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[sonichi]

If folks are OK with this change, a test should be added to cover the added case.

[sonichi]

@bitnom could you add a test to cover the new line added?

[bitnom]

@bitnom could you add a test to cover the new line added?

it is committed

[bitnom]

@bitnom could you add a test to cover the new line added?

should be good now