[otel_collector_internal_telemetry]: ESQL errors without otelcol_exporter_send_failed_spans

### Integration Name

OpenTelemetry Collector Internal Telemetry [packages/otel_collector_internal_telemetry]

### Dataset Name

_No response_

### Integration Version

doesn't matter

### Agent Version

doesn't matter

### Agent Output Type

elasticsearch

### Elasticsearch Version

doesn't matter

### OS Version and Architecture

doesn't matter

### Software/API Version

_No response_

### Error Message

Some of the dashboards show an error - specifically when `otelcol_exporter_send_failed_spans` isn't yet present.

`SET unmapped_fields="nullify";` would solve these in the queries.

Report and suggestion by @ty-elastic

### Event Original

_No response_

### What did you do?

Add integration and not have all the internal telemetry ingested yet - meaning some of the metrics are not (yet) present.

### What did you see?

Errors on some of the dashboards.

### What did you expect to see?

No error - empty charts are fine (as metrics may not be present), but not an error.

### Anything else?

_No response_