The ElementAttributeBuilder should return null instead of empty string to prevent Blazor from generating empty attributes on final html

[Cyrus-Sushiant]

When using ElementStyleBuilder in Blazor, if a value is empty, it currently returns an empty string. This causes Blazor to add an attribute with an empty value to the element, which is not the desired behavior. Instead, it should return null to ensure that no attribute is added when the value is empty.