fix(ui5-color-picker): add missing hex value to RGBtoHEX method

[hinzzx]

There was a HEX value missing in the RGBtoHEX() method in the ColorConversion util.
With this change we added it.

Before

<div id="text"></div>

<script>
document.getElementById("text").innerHTML = RGBtoHEX({
  r: 207,
  b: 223,
  g: 255
});  // results in #CundefinedundefinedundefinedDundefined
</script>

After

<div id="text"></div>

<script>
document.getElementById("text").innerHTML = RGBtoHEX({
  r: 207,
  b: 223,
  g: 255
});  // results in #CFFFDF
</script>

Fixes: #9895