Fix VMDTextField parameter not used in the implementation

[Vincent-Ribou]

Description

I added a check if a placeholder is passed to the function, if it is the case we use the placeholder instead of the Text.

Motivation and Context

In the VMDTextField we have a placeholder parameter, but it was not used in the implementation.
It was impossible to have a custom placeholder for the VMDText since what we passed to the TextField was a simple Text with a style. I ran in a case where my text input was on a single line, but the placeholder was on multiple lines. Being able to set a placeholder will solve this case and make the VMDTextField more flexible.

How Has This Been Tested?

I tested this change in a project. I tested the function by passing a placeholder as parameter.
I also tested without a parameter to make sure everything still work correctly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)