Fix for issue setting ClientSidePage taxonomy field values from parameters empty

[magarma]

We are having an issue ( #406 ) when we are creating a ClientSidePage with taxonomy field values and those are comming from parameters and are empty...
for example,

...
<pnp:Provisioning xmlns:pnp="http://schemas.dev.office.com/PnP/2020/02/ProvisioningSchema">
  <pnp:Preferences>
    <pnp:Parameters>
      <pnp:Parameter Key="parameterTest1"></pnp:Parameter>
    </pnp:Parameters>
  </pnp:Preferences>
...
<pnp:ClientSidePage PromoteAsNewsArticle="false" PromoteAsTemplate="false" Overwrite="true" EnableComments="false" Title="Home" ContentTypeID="0x010100...."  PageName="Home.aspx" Layout="Home">
...          
<pnp:FieldValues>
            <pnp:FieldValue Key="CPLocationGeography" Value="{parameter:parameterTest1}"/>
 <pnp:FieldValues>
 </pnp:ClientSidePage>
 ...

In ObjectClientSidePage.cs there is a call...

This is giving an exception because the code is only checking by null and not by empty and when calling to the methdo GetTaxonomyItemByPath with an empty string is raising the exception.
"Activity failed with an error. Reason: System.ArgumentNullException: Value cannot be null. (Parameter 'path')"

Sending PR to check also by string empty.