title: Material-ui/pickers: When InputAdornmentProps property with position start added to KeyboardDatePicker an unwanted margin appears
body: <p>I have this component from material-ui/pickers:</p>
  <pre><code>&lt;KeyboardDatePicker
     value={selectedDate}
     onChange={(_, newValue) =&gt; handleClick(newValue)}
     labelFunc={renderLabel}
     disableToolbar
     variant='inline'
     inputVariant='filled'
     format='YYYYMMDD'
     allowKeyboardControl
     autoOk
     inputProps={{
       'data-testid': `input-${name}`
     }}
     InputAdornmentProps={{ position: 'start' }}
   /&gt;
  </code></pre>
  <p>This property is causing the problem.
  Without it the style is fine an the icon is centered vertically in the component. When I try to move the icon to the beginning of the component there appears an extra style:</p>
  <pre><code>    .MuiInputAdornment-filled.MuiInputAdornment-positionStart:not(.MuiInputAdornment-hiddenLabel) {
     margin-top: 16px;
  }
  </code></pre>
  <p>That causes icon to be off-center.
  This is what it looks like without InputAdornmentProps:
  [1]: <a href="https://i.stack.imgur.com/xxGaM.png" rel="nofollow noreferrer">https://i.stack.imgur.com/xxGaM.png</a></p>
  <p>And with it:
  [2]: <a href="https://i.stack.imgur.com/dHA53.png" rel="nofollow noreferrer">https://i.stack.imgur.com/dHA53.png</a></p>
  <p>What could be causing it?</p>