### 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
var eval = as(limit.child(), Eval.class);
        assertEquals(1, eval.fields().size());
        Alias eeField = eval.fields().get(0);
        assertEquals("ee", eeField.name());
        assertEquals("e", ((ReferenceAttribute) eeField.child()).name());
        assertEquals(3, eval.output().size());
        Attribute empNo = eval.output().get(0);
        assertEquals("emp_no", empNo.name());
        assertThat(empNo, instanceOf(FieldAttribute.class));
        Attribute e = eval.output().get(1);
        assertEquals("e", e.name());
        assertThat(e, instanceOf(ReferenceAttribute.class));
        Attribute ee = eval.output().get(2);
        assertEquals("ee", ee.name());
        assertThat(ee, instanceOf(ReferenceAttribute.class));
    }
    public void testRowAttributeResolution() {
        EsIndex idx = new EsIndex("idx", Map.of());
        Analyzer analyzer = analyzer(IndexResolution.valid(idx));
        var plan = analyzer.analyze(
            new Eval(
                EMPTY,
                new Row(EMPTY, List.of(new Alias(EMPTY, "emp_no", new Literal(EMPTY, 1, DataTypes.INTEGER)))),
                List.of(new Alias(EMPTY, "e", new UnresolvedAttribute(EMPTY, "emp_no")))
            )
        );
        var limit = as(plan, Limit.class);
        var eval = as(limit.child(), Eval.class);
        assertEquals(1, eval.fields().size());
        assertEquals(new Alias(EMPTY, "e", new ReferenceAttribute(EMPTY, "emp_no", DataTypes.INTEGER)), eval.fields().get(0));