```
ROW a="b c d"
| DISSECT a "%{b} %{} %{d}"
```
returns

```
       a       |       b       |               |       d       
---------------+---------------+---------------+---------------
b c d          |b              |null           |d 
```

it should ignore the empty pattern instead, and only return `a, b, d`
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/parser/StatementParserTests.java
);
    }
    public void testMetadataFieldOnOtherSources() {
        expectError(
            "row a = 1 [metadata _index]",
            "1:11: mismatched input '[' expecting {<EOF>, PIPE, 'and', COMMA, 'or', '+', '-', '*', '/', '%'}"
        );
        expectError("show functions [metadata _index]", "line 1:16: mismatched input '[' expecting {<EOF>, PIPE}");
        expectError(
            "explain [from foo] [metadata _index]",
            "line 1:20: mismatched input '[' expecting {PIPE, COMMA, OPENING_BRACKET, ']'}"
        );
    }
    public void testMetadataFieldMultipleDeclarations() {
        expectError("from test [metadata _index, _version, _index]", "1:40: metadata field [_index] already declared [@1:21]");
    }
    public void testMetadataFieldUnsupportedPrimitiveType() {
        expectError("from test [metadata _tier]", "line 1:22: unsupported metadata field [_tier]");
    }
    public void testMetadataFieldUnsupportedCustomType() {
        expectError("from test [metadata _feature]", "line 1:22: unsupported metadata field [_feature]");
    }
    public void testMetadataFieldNotFoundNonExistent() {
        expectError("from test [metadata _doesnot_compute]", "line 1:22: unsupported metadata field [_doesnot_compute]");
    }
    public void testMetadataFieldNotFoundNormalField() {
        expectError("from test [metadata emp_no]", "line 1:22: unsupported metadata field [emp_no]");
    }
    public void testDissectPattern() {
        LogicalPlan cmd = processingCommand("dissect a \"%{foo}\"");