In process_check_run, serialize current transactions if null

[agritheory]

This happens when the configured settings for a Check Run allow it to be processed without any other changes.

There are a couple of events that lead to this error, and it would be better to address the root causes:

• get_entries is called from the browser but frm is not marked as dirty at this point even though the data in transactions has changed
• The document isn't aware that it cannot be processed because there are no transactions

To reproduce:
bench reinstall --yes --admin-password admin --mariadb-root-password admin && bench execute 'check_run.tests.setup.before_test'

Then start a new Check Run with the default company, account and bank account.
Then "Modify Settings" and check "Pre-Select Overdue Items" and "Automatically Release On Hold Invoices"
Return to the Check Run and click "Process Check Run"

Stack Trace

### App Versions ``` { "check_run": "14.11.5", "erpnext": "14.70.6", "frappe": "14.76.2", "hrms": "14.28.5" } ``` ### Route ``` Form/Check Run/ACC-CR-2024-00001 ``` ### Traceback ``` Traceback (most recent call last): File "apps/frappe/frappe/app.py", line 97, in application response = frappe.api.handle() File "apps/frappe/frappe/api.py", line 55, in handle return frappe.handler.handle() File "apps/frappe/frappe/handler.py", line 48, in handle data = execute_cmd(cmd) File "apps/frappe/frappe/handler.py", line 86, in execute_cmd return frappe.call(method, **frappe.form_dict) File "apps/frappe/frappe/__init__.py", line 1619, in call return fn(*args, **newargs) File "apps/check_run/check_run/check_run/doctype/check_run/check_run.py", line 939, in process_check_run doc.process_check_run() File "apps/check_run/check_run/check_run/doctype/check_run/check_run.py", line 188, in process_check_run for t in json.loads(self.transactions) File "/home/tyler/.pyenv/versions/3.10.10/lib/python3.10/json/__init__.py", line 339, in loads raise TypeError(f'the JSON object must be str, bytes or bytearray, ' TypeError: the JSON object must be str, bytes or bytearray, not NoneType

### Request Data

{
"type": "POST",
"args": {
"docname": "ACC-CR-2024-00001"
},
"headers": {},
"error_handlers": {},
"url": "/api/method/check_run.check_run.doctype.check_run.check_run.process_check_run"
}

### Response Data

{
"exception": "TypeError: the JSON object must be str, bytes or bytearray, not NoneType"
}

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