Trigger validation on click of Search button

I'm building an MVVM Light WPF app using Visual Studio 2015 Update 1. I have the following two search fields: cmbSearchColumn and txtSearchValue. Neither can be blank when the user clicks the Search button. Note that I've got ValidationRules set for both fields.

Here's the relevant XAML:

<TextBlock Grid.Row="1"
           Grid.Column="0"
           Style="{StaticResource FieldLabel}">
    Search Column
</TextBlock>
<StackPanel Grid.Row="1"
            Grid.Column="1"
            Style="{StaticResource ValidationStackPanel}">
    <ComboBox x:Name="cmbSearchColumn"
              DisplayMemberPath="MemberName"
              IsEditable="True"
              ItemsSource="{Binding SearchColumns}"
              SelectedValuePath="MemberValue"
              Style="{StaticResource ComboBoxStyle}">
        <ComboBox.SelectedItem>
            <Binding Mode="TwoWay"
                     Path="SelectedColumn}"
                     UpdateSourceTrigger="Explicit">
                <Binding.ValidationRules>
                    <helpers:NotEmptyStringValidationRule 
  Message="Search Column cannot be blank." ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </ComboBox.SelectedItem>
    </ComboBox>
    <TextBlock Style="{StaticResource FieldLabelError}" 
  Text="{Binding (Validation.Errors)[0].ErrorContent, ElementName=cmbSearchColumn}" />
</StackPanel>
<TextBlock Grid.Row="2"
           Grid.Column="0"
           Padding="0 0 9 9"
           Style="{StaticResource FieldLabel}">
    Search Value
</TextBlock>
<StackPanel Grid.Row="1"
            Grid.Column="1"
            Style="{StaticResource ValidationStackPanel}">
    <TextBox x:Name="txtSearchValue" Style="{StaticResource FieldTextBox}">
        <TextBox.Text>
            <Binding Mode="TwoWay"
                     Path="SearchValue"
                     UpdateSourceTrigger="Explicit">
                <Binding.ValidationRules>
                    <helpers:NotEmptyStringValidationRule 
  Message="Search Value cannot be blank." ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
    <TextBlock Style="{StaticResource FieldLabelError}" 
  Text="{Binding (Validation.Errors)[0].ErrorContent, ElementName=txtSearchValue}" />
</StackPanel>   
<Button Grid.Row="4"
    Grid.Column="1"
    Command="{Binding SearchEmployeesRelayCommand}"
    Content="Search"
    Style="{StaticResource FieldButton}" />

When the app loads, it immediately displays the error next to the fields, saying that they cannot be blank. However, I need to trigger the validation on them only when the user clicks the Search button.

How do I do this? Thanks.