Fix null reference in table formatting

[gregli-msft]

Without this change:

• With table formatting on, it is hard to distinguish a blank record from a blank field in a table. In the example below, a blank record is represented by a Blank() which could be confused for the value of the first column (which shows an empty space for Blank() which could be confusing).
• With table formatting off, it encounters a null reference exception.

>> Table(Blank(),{a:Blank(),b:true},Blank(),{a:42,b:Blank()})

  a    b
 ==== ======
 Blank()
       true
 Blank()
  42

>> Option( Options.FormatTable, false )
false

>> Table(Blank(),{a:Blank(),b:true},Blank(),{a:42,b:Blank()})
Unhandled exception. System.AggregateException: One or more errors occurred. (Object reference not set to an instance of an object.)
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.PowerFx.Repl.Services.StandardFormatter.TryGetSpecialFieldNames(RecordValue record) in D:\repos\pfx-udt-decimal\src\libraries\Microsoft.PowerFx.Repl\Services\StandardFormatter.cs:line 100
   at Microsoft.PowerFx.Repl.Services.StandardFormatter.FormatRecordCore(RecordValue record) in D:\repos\pfx-udt-decimal\src\libraries\Microsoft.PowerFx.Repl\Services\StandardFormatter.cs:line 37
   at Microsoft.PowerFx.Repl.Services.StandardFormatter.FormatStandardTable(TableValue table, HashSet`1 selectedFieldNames) in D:\repos\pfx-udt-decimal\src\libraries\Microsoft.PowerFx.Repl\Services\StandardFormatter.cs:line 294
...

After this change:

>> Table(Blank(),{a:Blank(),b:true},Blank(),{a:42,b:Blank()})

  a    b
 ==== ======
 <blank record>
       true
 <blank record>
  42

>> Option( Options.FormatTable, false )
false

>> Table(Blank(),{a:Blank(),b:true},Blank(),{a:42,b:Blank()})
[Blank(), {a:Blank(), b:true}, Blank(), {a:42, b:Blank()}]