### 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/test/java/org/elasticsearch/xpack/esql/analysis/AnalyzerTests.java
public void testUnsupportedValidFieldTypeInNestedParentField() {
        verifyUnsupported("""
            from test
            | keep dep.dep_id.keyword
            """, "Found 1 problem\n" + "line 2:8: Unknown column [dep.dep_id.keyword]", "mapping-multi-field-with-nested.json");
    }
    public void testUnsupportedObjectAndNested() {
        verifyUnsupported(
            """
                from test
                | keep dep, some
                """,
            "Found 2 problems\n" + "line 2:8: Unknown column [dep]\n" + "line 2:13: Unknown column [some]",
            "mapping-multi-field-with-nested.json"
        );
    }
    public void testDropNestedField() {
        verifyUnsupported(
            """
                from test
                | drop dep, dep.dep_id.keyword
                """,
            "Found 2 problems\n" + "line 2:8: Unknown column [dep]\n" + "line 2:13: Unknown column [dep.dep_id.keyword]",
            "mapping-multi-field-with-nested.json"
        );
    }
    public void testDropNestedWildcardField() {
        verifyUnsupported("""
            from test
            | drop dep.*