[text field] Fix outline gap without a label in Firefox

Firefox applies the inherited letter-spacing to the zero-width space in
the unlabeled legend, which leaves a 0.15px gap in the border. Cap the
legend at `maxWidth: 0` when there is no label.

packages/mui-material/src/OutlinedInput/NotchedOutline.js

@@ -39,6 +39,7 @@ const NotchedOutlineLegend = styled('legend', {
         style: {
           padding: 0,
           lineHeight: '11px', // sync with `height` in `legend` styles
+          maxWidth: 0,
           ...getTransitionStyles(theme, 'width', {
             duration: 150,
             easing: theme.transitions.easing.easeOut,

packages/mui-material/src/OutlinedInput/NotchedOutline.test.js

@@ -64,4 +64,11 @@ describe('<NotchedOutline />', () => {
       });
     },
   );
+
+  it.skipIf(isJsdom())('should not leave a gap in the outline without a label', () => {
+    const { container } = render(
+      <NotchedOutline notched={false} style={{ letterSpacing: '0.00938em' }} />,
+    );
+    expect(container.querySelector('legend').getBoundingClientRect().width).to.equal(0);
+  });
 });