```
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
assertEquals(Dissect.class, cmd.getClass());
        Dissect dissect = (Dissect) cmd;
        assertEquals("%{foo}", dissect.parser().pattern());
        assertEquals("", dissect.parser().appendSeparator());
        assertEquals(List.of(referenceAttribute("foo", KEYWORD)), dissect.extractedFields());
        for (String separatorName : List.of("append_separator", "APPEND_SEPARATOR", "AppEnd_SeparAtor")) {
            cmd = processingCommand("dissect a \"%{foo}\" " + separatorName + "=\",\"");
            assertEquals(Dissect.class, cmd.getClass());
            dissect = (Dissect) cmd;
            assertEquals("%{foo}", dissect.parser().pattern());
            assertEquals(",", dissect.parser().appendSeparator());
            assertEquals(List.of(referenceAttribute("foo", KEYWORD)), dissect.extractedFields());
        }
        for (Tuple<String, String> queryWithUnexpectedCmd : List.of(
            Tuple.tuple("from a | dissect foo \"\"", "[]"),
            Tuple.tuple("from a | dissect foo \" \"", "[ ]"),
            Tuple.tuple("from a | dissect foo \"no fields\"", "[no fields]")
        )) {
            expectError(queryWithUnexpectedCmd.v1(), "Invalid pattern for dissect: " + queryWithUnexpectedCmd.v2());
        }
        expectError("from a | dissect foo \"%{*a}:%{&a}\"", "Reference keys not supported in dissect patterns: [%{*a}]");
        expectError("from a | dissect foo \"%{bar}\" invalid_option=3", "Invalid option for dissect: [invalid_option]");
        expectError(
            "from a | dissect foo \"%{bar}\" append_separator=3",
            "Invalid value for dissect append_separator: expected a string, but was [3]"
        );
    }
    public void testGrokPattern() {
        LogicalPlan cmd = processingCommand("grok a \"%{WORD:foo}\"");
        assertEquals(Grok.class, cmd.getClass());