### Elasticsearch Version

8.11.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

MacOS

### Problem Description

When sending a ESQL query with the `enrich` command that involves a policy that does not existing a `NullPointerException` is thrown and shown as error messages to the user:

Query:
```
row a  = 1| enrich missing-policy
```

Returns:
```
null_pointer_exception - Cannot invoke "org.elasticsearch.xpack.core.enrich.EnrichPolicy.getMatchField()" because "policy" is null
```

### Steps to Reproduce

See above.

### Logs (if relevant)

_No response_
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Analyzer.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.esql.analysis;
import org.elasticsearch.common.regex.Regex;
import org.elasticsearch.common.util.set.Sets;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.xpack.core.enrich.EnrichPolicy;
import org.elasticsearch.xpack.esql.enrich.EnrichPolicyResolution;
import org.elasticsearch.xpack.esql.expression.function.UnsupportedAttribute;
import org.elasticsearch.xpack.esql.plan.logical.Drop;
import org.elasticsearch.xpack.esql.plan.logical.Enrich;
import org.elasticsearch.xpack.esql.plan.logical.EsqlUnresolvedRelation;