[thci] drop empty lines returned in command outout from the response (#7236)

In some cases the first element from the output from __executeCommand
might be empty line, which casues command failures like on below
trace:

__executeCommand('extaddr',) returns ['', '166e0a0000000001', '', 'Done']
FUNC getMAC failed: invalid literal for int() with base 16: ''

This commit filters out empty string from the result of
__executeCommand.

tools/harness-thci/OpenThread.py

@@ -298,7 +298,9 @@ class OpenThreadTHCI(object):
                     continue
 
                 self.log("readline: %s", line)
-                response.append(line)
+                # skip empty lines
+                if line:
+                    response.append(line)
 
                 if line.endswith('Done'):
                     break