### 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
}
        public static List<NamedExpression> calculateEnrichFields(
            Source source,
            String policyName,
            List<Attribute> mapping,
            List<NamedExpression> enrichFields,
            EnrichPolicy policy
        ) {
            Map<String, Attribute> fieldMap = mapping.stream().collect(Collectors.toMap(NamedExpression::name, Function.identity()));
            fieldMap.remove(policy.getMatchField());
            List<NamedExpression> result = new ArrayList<>();
            if (enrichFields == null || enrichFields.isEmpty()) {
                
                for (String enrichFieldName : policy.getEnrichFields()) {
                    result.add(createEnrichFieldExpression(source, policyName, fieldMap, enrichFieldName));
                }
            } else {
                for (NamedExpression enrichField : enrichFields) {
                    String enrichFieldName = Expressions.name(enrichField instanceof Alias a ? a.child() : enrichField);
                    NamedExpression field = createEnrichFieldExpression(source, policyName, fieldMap, enrichFieldName);
                    result.add(enrichField instanceof Alias a ? new Alias(a.source(), a.name(), field) : field);
                }
            }
            return result;
        }
        private static NamedExpression createEnrichFieldExpression(
            Source source,
            String policyName,
            Map<String, Attribute> fieldMap,
            String enrichFieldName