Changes

http_json_api: Only require Content-Type if response body is non-empty

If there is an empty body returned, such as in the case of the response
to an es9p notification, then it is of course also legal to not set the
content-type header.

This patch fixes an exception when talking to certain SM-DP+ with
es9p_client.py:

DEBUG:pySim.esim.http_json_api:HTTP RSP-STS: [204] hdr: {'X-Admin-Protocol': 'gsma/rsp/v2.5.0', 'Date': 'Wed, 28 Jan 2026 18:26:39 GMT', 'Server': 'REDACTED'}
DEBUG:pySim.esim.http_json_api:HTTP RSP: b''
{'X-Admin-Protocol': 'gsma/rsp/v2.5.0', 'Date': 'Wed, 28 Jan 2026 18:26:39 GMT', 'Server': 'REDACTED'}
<Response [204]>
Traceback (most recent call last):
  File "gprojects/git/pysim/es9p/../contrib/es9p_client.py", line 315, in <module>
    c.do_notification()
    ~~~~~~~~~~~~~~~~~^^
  File "projects/git/pysim/es9p/../contrib/es9p_client.py", line 159, in do_notification
    res = self.peer.call_handleNotification(data)
  File "projects/git/pysim/contrib/pySim/esim/es9p.py", line 174, in call_handleNotification
    return self.handleNotification.call(data)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "projects/git/pysim/contrib/pySim/esim/http_json_api.py", line 335, in call
    if not response.headers.get('Content-Type').startswith(req_headers['Content-Type']):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'startswith'

Change-Id: I99e8f167b7bb869c5ff6d908ba673dac87fef71a