software/caddy/test: prevent tracebacks during tests

In the test output, there are lots of tracebacks like this one:

    "./software/caddy-frontend/test/test.py", line 534, in do_GET
        for header in config.pop('X-Drop-Header', '').split():
    AttributeError: 'NoneType' object has no attribute 'split'

this does not seem to impact the test result, but it makes the result hard to
read.

software/caddy-frontend/test/test.py

@@ -531,12 +531,12 @@ class TestHandler(BaseHTTPRequestHandler):
       timeout = int(config.pop('Timeout', '0'))
       compress = int(config.pop('Compress', '0'))
       drop_header_list = []
-      for header in config.pop('X-Drop-Header', '').split():
+      for header in (config.pop('X-Drop-Header') or '').split():
         drop_header_list.append(header)
       header_dict = config
     else:
       drop_header_list = []
-      for header in self.headers.dict.get('x-drop-header', '').split():
+      for header in (self.headers.dict.get('x-drop-header') or '').split():
         drop_header_list.append(header)
       response = None
       status_code = 200