Update NetTraceFormat.md (#2273)

Add missing info about LabelList ID 0 being the empty list

Author: noahfalk

src/TraceEvent/EventPipe/NetTraceFormat.md

@@ -446,7 +446,7 @@ The LabelList block, BlockHeader.Kind=8, contains a set of key-value pairs that
 
 The content of a LabelListBlock is:
 
-- firstIndex - uint32  // The index of the first entry in the block. Each successive entry is implicitly indexed by the previous entry's index + 1.
+- firstIndex - uint32  // The index of the first entry in the block. Each successive entry is implicitly indexed by the previous entry's index + 1. firstIndex must be >= 1.
 - count - uint32       // The number of entries in the block.
 - Concatenated sequence of label_lists, each of which is:
   - one or more Label entries each of which is:
@@ -471,6 +471,8 @@ If the high bit of the Kind field is set that demarcates that this is the last l
 Similar to StackBlock, references to a row in the LabelListBlock are only valid in the file after the LabelListBlock that defines it and before the next SequencePoint block.
 This prevents the reader from needing a lookup table that grows indefinitely with file length or requiring the reader to search the entire file to resolve a given label list index.
 
+An empty LabelList can't be explicitly encoded in the LabelListBlock, but implicitly the LabelList index 0 refers to an empty LabelList. 
+
 ## Changes relative to older file format versions
 
 ### Versions 1-3
@@ -556,4 +558,4 @@ There was no way to add new optional fields that are independent of the event ty
 In particular OpenTelemetry has embraced the W3C TraceContext standard for distributed tracing and I'd like to be able to use that as a common correlation identifier in the future.
 
 1. Adds a new LabelListBlock to store key-value pairs that can be referenced by index within the EventHeader.
-2. Removes the ActivityId and RelatedActivityId fields from the EventHeader and replaces them with a LabelListIndex field. Activity ids can be stored within the LabelList.
+2. Removes the ActivityId and RelatedActivityId fields from the EventHeader and replaces them with a LabelListIndex field. Activity ids can be stored within the LabelList.