### 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
"int",
            "keyword",
            "point",
            "shape",
            "some.ambiguous",
            "some.ambiguous.normalized",
            "some.ambiguous.one",
            "some.ambiguous.two",
            "some.dotted.field",
            "some.string",
            "some.string.normalized",
            "some.string.typical",
            "text",
            "unsigned_long",
            "version"
        );
    }
    public void testDropMultipleUnsupportedFieldsExplicitly() {
        verifyUnsupported("""
            from test
            | drop languages, gender
            """, "Unknown column [languages]");
    }
    public void testDropPatternUnsupportedFields() {
        assertProjection("""
            from test
            | drop *ala*
            """, "_meta_field", "emp_no", "first_name", "gender", "job", "job.raw", "languages", "last_name", "long_noidx");
    }
    public void testDropUnsupportedPattern() {