gdb/mi: handle no condition argument case for -break-condition

gdb/mi: handle no condition argument case for -break-condition

As reported in PR gdb/28076 [1], passing no condition argument to the
-break-condition command (e.g.: "-break-condition 2") should clear the
condition for breakpoint 2, just like CLI's "condition 2", but instead
an error message is returned:

  ^error,msg="-break-condition: Missing the <number> and/or <expr> argument"

The current implementation of the -break-condition command's argument
handling (79aabb7308c "gdb/mi: add a '--force' flag to the
'-break-condition' command") was done according to the documentation,
where the condition argument seemed mandatory.  However, the
-break-condition command originally (i.e. before the 79aabb7308c
patch) used the CLI's "cond" command, and back then not passing a
condition argument was clearing out the condition.  So, this is a
regression in terms of the behavior.

Fix the argument handling of the -break-condition command to allow not
having a condition argument, and also update the document to make the
behavior clear.  Also add test cases to test the scenarios which were
previously not covered.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=28076