Ensure @custom-variant foo () has a non-empty selector list (#16009)

This PR fixes an issue where an empty selector list was valid when
defining a `@custom-variant`.

Given this input:
```css
@custom-variant foo ();
```

If you then use it with a utility such as `foo:flex`, then the following
(incorrect) CSS was generated:

```css
.foo\:flex {
   {
    display: flex;
  }
}
```

Which is invalid CSS. This PR will now validate that that we have at
least _something_ and show an error accordingly.

Author: RobinMalfait

Diff for: CHANGELOG.md

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Find utilities when using functions inside arrays ([#15974](https://github.com/tailwindlabs/tailwindcss/pull/15974))
 - Ensure that `@tailwindcss/browser` does not pollute the global namespace ([#15978](https://github.com/tailwindlabs/tailwindcss/pull/15978))
 - Fix crash when project lives in the `/` directory ([#15988](https://github.com/tailwindlabs/tailwindcss/pull/15988))
+- Ensure `@custom-variant` has a non-empty selector list  ([#16009](https://github.com/tailwindlabs/tailwindcss/pull/16009))
 - _Upgrade_: Ensure JavaScript config files on different drives are correctly migrated ([#15927](https://github.com/tailwindlabs/tailwindcss/pull/15927))
 - _Upgrade_: Migrate `leading-[1]` to `leading-none` ([#16004](https://github.com/tailwindlabs/tailwindcss/pull/16004))
 - _Upgrade_: Do not migrate arbitrary leading utilities to bare utilities ([#16004](https://github.com/tailwindlabs/tailwindcss/pull/16004))

Diff for: packages/tailwindcss/src/index.test.ts

@@ -2514,6 +2514,26 @@ describe('@custom-variant', () => {
     ).rejects.toThrowErrorMatchingInlineSnapshot(`[Error: \`@custom-variant\` cannot be nested.]`)
   })
 
+  test('@custom-variant must not have an empty selector', () => {
+    return expect(
+      compileCss(css`
+        @custom-variant foo ();
+      `),
+    ).rejects.toThrowErrorMatchingInlineSnapshot(
+      `[Error: \`@custom-variant foo ()\` selector is invalid.]`,
+    )
+  })
+
+  test('@custom-variant with multiple selectors, cannot be empty', () => {
+    return expect(
+      compileCss(css`
+        @custom-variant foo (.foo, .bar, );
+      `),
+    ).rejects.toThrowErrorMatchingInlineSnapshot(
+      `[Error: \`@custom-variant foo (.foo, .bar, )\` selector is invalid.]`,
+    )
+  })
+
   test('@custom-variant with no body must include a selector', () => {
     return expect(
       compileCss(css`

Diff for: packages/tailwindcss/src/index.ts

@@ -281,6 +281,11 @@ async function parseCss(
         }
 
         let selectors = segment(selector.slice(1, -1), ',')
+        if (selectors.length === 0 || selectors.some((selector) => selector.trim() === '')) {
+          throw new Error(
+            `\`@custom-variant ${name} (${selectors.join(',')})\` selector is invalid.`,
+          )
+        }
 
         let atRuleParams: string[] = []
         let styleRuleSelectors: string[] = []