fix: Set customer only if contact is present

[surajshetty3416]

to avoid

Traceback (most recent call last):
  File "/home/frappe/frappe-io-bench/apps/erpnext/erpnext/erpnext_integrations/exotel_integration.py", line 22, in handle_incoming_call
    create_call_log(call_payload)
  File "/home/frappe/frappe-io-bench/apps/erpnext/erpnext/erpnext_integrations/exotel_integration.py", line 64, in create_call_log
    call_log.save(ignore_permissions=True)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 285, in save
    return self._save(*args, **kwargs)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 307, in _save
    self.insert()
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 231, in insert
    self.run_method("before_insert")
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 831, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 1116, in composer
    return composed(self, method, *args, **kwargs)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 1099, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 825, in <lambda>
    fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
  File "/home/frappe/frappe-io-bench/apps/erpnext/erpnext/communication/doctype/call_log/call_log.py", line 19, in before_insert
    contact = frappe.get_doc("Contact", self.contact)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/__init__.py", line 794, in get_doc
    doc = frappe.model.document.get_doc(*args, **kwargs)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 75, in get_doc
    return controller(*args, **kwargs)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 113, in __init__
    self.load_from_db()
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/model/document.py", line 156, in load_from_db
    frappe.throw(_("{0} {1} not found").format(_(self.doctype), self.name), frappe.DoesNotExistError)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/__init__.py", line 413, in throw
    msgprint(msg, raise_exception=exc, title=title, indicator='red', is_minimizable=is_minimizable, wide=wide)
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/__init__.py", line 392, in msgprint
    _raise_exception()
  File "/home/frappe/frappe-io-bench/apps/frappe/frappe/__init__.py", line 340, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.DoesNotExistError: Contact None not found

while creation of call log