Hi you can try like this,
<asp:label id="maillbl" runat="server" visible='<%# Eval("ContactMail").ToString()!=""?true:false %>' text="<%# Eval("ContactMail")%>" xmlns:asp="#unknown">
</asp:label>
To hide the image button
<asp:ImageButton ID="ImageButton1" Visible='<%# Eval("ContactMail")!=""?true:false %>' OnClick="ImageButton1_Click" CommandArgument='<%# Eval("WalkinID")%>' ImageUrl="~/images/apply.png" runat="server" />