fix: ignore virtual doctypes during data export

[Alchez]

Trying to export records for doctypes that have a child table as a virtual doctype raises the following error:

ProgrammingError: (1146, "Table '_0ece7759525955dd.tab{redacted}' doesn't exist")
  File "frappe/app.py", line 69, in application
    response = frappe.api.handle()
  File "frappe/api.py", line 54, in handle
    return frappe.handler.handle()
  File "frappe/handler.py", line 45, in handle
    data = execute_cmd(cmd)
  File "frappe/handler.py", line 83, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "__init__.py", line 1585, in call
    return fn(*args, **newargs)
  File "frappe/core/doctype/data_export/exporter.py", line 61, in export_data
    exporter.build_response()
  File "frappe/core/doctype/data_export/exporter.py", line 141, in build_response
    self.add_data()
  File "frappe/core/doctype/data_export/exporter.py", line 379, in add_data
    for ci, child in enumerate(data_row.run(as_dict=True)):
  File "frappe/query_builder/utils.py", line 76, in execute_query
    return frappe.db.sql(query, params, *args, **kwargs)  # nosemgrep
  File "frappe/database/database.py", line 206, in sql
    self._cursor.execute(query, values)
  File "pymysql/cursors.py", line 148, in execute
    result = self._query(query)
  File "pymysql/cursors.py", line 310, in _query
    conn.query(q)
  File "pymysql/connections.py", line 548, in query
    self._affected_rows = self._read_query_result(unbuffered=unbuffered)
  File "pymysql/connections.py", line 775, in _read_query_result
    result.read()
  File "pymysql/connections.py", line 1156, in read
    first_packet = self.connection._read_packet()
  File "pymysql/connections.py", line 725, in _read_packet
    packet.raise_for_error()
  File "pymysql/protocol.py", line 221, in raise_for_error
    err.raise_mysql_exception(self._data)
  File "pymysql/err.py", line 143, in raise_mysql_exception
    raise errorclass(errno, errval)

In another iteration, we could use a common controller method to add the rows instead.

[codecov]

Codecov Report

Merging #20891 (2517648) into develop (4b7c735) will increase coverage by 0.00%.
The diff coverage is 66.66%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #20891   +/-   ##
========================================
  Coverage    63.89%   63.89%           
========================================
  Files          761      761           
  Lines        68821    68824    +3     
  Branches      6219     6219           
========================================
+ Hits         43970    43975    +5     
+ Misses       21309    21307    -2     
  Partials      3542     3542           

Flag	Coverage Δ
server	68.92% <66.66%> (-0.01%)	⬇️

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