Value '' cannot be converted

'' is not a value: it's a "character literal" without a character inside it. A bit like saying "a cup of coffee" without the cup!

You need a character specifier inside the quotes, like this perhaps which may insert a "null character" (not the same as a null value):

TargetNullValue = '\0'

<TextBox Text="{Binding SomeProperty, TargetNullValue='No value set here...'}" />