1. How do you want to use this partial index?

2. NULL values are indexed, same as any other value (with one difference that any number of NULL values are allowed in  a Unique index). Consider the following example:

Class Sample.Person Extends %Persistent
{

Property Name As %String;

Index NameIndex On Name;

/// do ##class(Sample.Person).Test()
ClassMethod Test()
{
    kill ^Sample.PersonD, ^Sample.PersonI
    do ..AddPerson("ed")
    do ..AddPerson("ed")
    do ..AddPerson("bob")
    do ..AddPerson()
    do ..AddPerson()
    
    zw ^Sample.PersonD, ^Sample.PersonI
}

ClassMethod AddPerson(Name)
{
    set p = ..%New()
    set:$d(Name) p.Name = Name
    do p.%Save()
}

Whn I run the text method:

do ##class(Sample.Person).Test()

I get the following data global:

^Sample.PersonD=5
^Sample.PersonD(1)=$lb("","ed")
^Sample.PersonD(2)=$lb("","ed")
^Sample.PersonD(3)=$lb("","bob")
^Sample.PersonD(4)=$lb("","")
^Sample.PersonD(5)=$lb("","")

And the following index global:

^Sample.PersonI("NameIndex"," ",4)=""
^Sample.PersonI("NameIndex"," ",5)=""
^Sample.PersonI("NameIndex"," BOB",3)=""
^Sample.PersonI("NameIndex"," ED",1)=""
^Sample.PersonI("NameIndex"," ED",2)=""

As you see NULL values are indexed same as any other value.

Index global has the following structure:

^ClassIndexGlobal(IndexName, IndexValue, Id) = DataStoredInIndex

Where:

• ClassIndexGlobal - is a global  name used for storing class globals. Defined in Storage.
• IndexName-  is a name of the index
• IndexValue - is a collated stored value (so "bob" becomes " BOB", and NULL  becomes " "). Some additional info on collation. Documentation.
• Id - object id
• DataStoredInIndex - any additional data stored in index. Documenatation.

Also for this SQL:

SELECT ID
FROM Sample.Person
WHERE Name IS NULL

The following plan that uses our index gets generated:

• Read index map Sample.Person.NameIndex, using the given %SQLUPPER(Name), and looping on ID.
• For each row:
  •  Output the row.

You need to query Ens.MessageHeader class.

Here's an example of calling class method with two parameters:

<call method="Method">
  <parameter expression="%request.Get("URLPAPAM")"/>
  <parameter value="123"/>
</call>

Check %ZEN.Report.call and %ZEN.Report.parameter for details.

You can define a parameter as an ObjectScript expression that it is evaluated at runtime. To do so, specify its type as COSEXPRESSION and specify an ObjectScript expression as the value:

Parameter PARAMNAME As COSEXPRESSION = "ObjectScriptExpression";

where PARAMNAME is the parameter being defined and ObjectScriptExpression is the ObjectScript content that is evaluated at runtime.
An example class parameter definition would be:

Parameter DateParam As COSEXPRESSION = "$H";

Documentation.

That said, I'd recommend gradual refactoring of these parameters into methods.