Fix command escape bug in lldb-dap (#72902)

https://github.com/llvm/llvm-project/pull/69238 caused breakage in
VSCode debug console usage -- the user's input is always treated as
commands instead of expressions (the same behavior as if empty command
escape prefix is specified).

The bug is in one overload of `GetString()` which did not respect the
default value of "\`". But more important, I am puzzled to find out why
the regression is not caught by lldb test (testdap_evaluate). Turns out
https://github.com/llvm/llvm-project/pull/69238

 specifies
commandEscapePrefix default value in test framework to be "\`" while
VSCode will default not specify any commandEscapePrefix at all. Changing
it to None will fail `testdap_evaluate`. We should align the default
behavior between DAP client and testcase.

This patches fixes the bug in `GetString()` and changed the default
value of commandEscapePrefix in testcases to be None (be consistent with
IDE).

Co-authored-by: jeffreytan81 <jeffreytan@fb.com>