[Log Parsing Pipelines] qbtoexpr conversion breaks for string operators when targeting a nil attribute

[raj-k-singh]

Using a filter like log_file_name REGEX ^access* . The simulation is fine but on deploying this is the error we get.

2023-11-06T15:50:06.047Z	warn	router/router.go:119	Running expression returned an error	{"kind": "processor", "name": "logstransform/pipeline_NginxAccesslogs", "pipeline": "logs", "operator_id": "router_signoz", "operator_type": "router", "error": "interface conversion: interface {} is nil, not string (1:26)\n | attributes.log_file_name matches \"^access*\"\n | .........................^"}