Filter out messages with empty body text (#7341)

Addresses #7271 

Anthropic claude models seem to have trouble with messags with empty
body text, this
change filters them out before sending a batch of messages.

### Release Notes
#### New Features
- N/A

#### Bug Fixes
- N/A


### QA Notes

---------

Signed-off-by: Pete <1178561+petetronic@users.noreply.github.com>
Co-authored-by: Wasim Lorgat <mwlorgat@gmail.com>

Author: petetronic

.vscode-test.js

@@ -63,6 +63,11 @@ const extensions = [
 		mocha: { timeout: 60_000 }
 	},
 	// --- Start Positron ---
+	{
+		label: 'positron-assistant',
+		workspaceFolder: path.join(os.tmpdir(), `positron-assistant-${Math.floor(Math.random() * 100000)}`),
+		mocha: { timeout: 60_000 }
+	},
 	{
 		label: 'positron-code-cells',
 		workspaceFolder: path.join(os.tmpdir(), `positron-code-cells-${Math.floor(Math.random() * 100000)}`),