How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values?

You could make a simple wrapper function, like

<%!
String blanknull(String s) {
  return (s == null) ? "" : s;
}
%>

then use it inside your JSP form, like

<input type="text" name="shoesize" value="<%=blanknull(shoesize)%>">