Fix: crash when using --json without --data on URLs without query params

[xyaz1313]

Problem

When running XSStrike with --json flag against a URL without query parameters, the tool crashes with:

AttributeError: 'str' object has no attribute 'keys'

Reproduction:

python xsstrike.py -u https://example.net --json

Root Cause

In xsstrike.py line 141-143:

elif jsonData:
    headers['Content-type'] = 'application/json'
    paramData = converter(paramData)  # paramData is None here

When paramData is None and jsonData is True:

1. converter(None) calls json.dumps(None) → returns "null" (a string)
2. getParams() sees jsonData is set, does params = data → params = "null"
3. scan() line 53: list(params.keys())[0] → crashes because strings don't have .keys()

Fix

Only convert paramData when it's actually provided:

elif jsonData and paramData:

This way, when --json is used without --data, paramData stays None, getParams() correctly returns None, and the "No parameters to test" error message is shown gracefully.

Fixes #438