Revert "Revert "Send DAP events through DDS"" - only check for event handler when DDS URI is also set.

Original change reverted due to test failure: https://github.com/dart-lang/sdk/issues/43743#issuecomment-1601278402

This reverts commit 02b10e1321d0d752cd162982140234ec092d685e.

Change-Id: Idb2cbffe18342c76d0cc062e5855c10a6df0e8f1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310780
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Helin Shiah <helinx@google.com>

https://dart.googlesource.com/sdk/+/5b3f7990e8db15dae976f1e1d4f119b73701c12c