Update chatdbg_pdb.py

Revert `line[0]` back to `line[:1]`.  The former crashes on an empty line; the latter is the empty string.  Could refactor but don't want to make unnecessary changes...

src/chatdbg/chatdbg_pdb.py

@@ -267,10 +267,10 @@ def _capture_onecmd(self, line):
             self.lastcmd = lastcmd
 
     def default(self, line):
-        if line[0] == "!":
+        if line[:1] == "!":
             super().default(line)
         else:
-            if line[0] == ":":
+            if line[:1] == ":":
                 line = line[1:].strip()
             self.do_chat(line)