Change per Issue #217 recommendations (#218)

* Empty Collection Recommendations based on #217


Service providers are unclear if they should return 404s when collection searches have no results. Per the advice in the issue 217 thread the recommendation is to NOT return a 404 in these cases and instead maintain consistency in their response and signifying no results as _value_  containing an empty array.

"We would always say that should return a 200 with an empty collection. The call succeeded, the collection exists. There's just nothing in it.

Given we always follow the convention of having the array itself be a 'value' property on a top-level object representing the collection itself, there's also the ability to return collection metadata that is valid regardless of whether there is anything in the collection, e.g. MaxItems." -- Gareth Jones

* TOC formatting

moving to a new section. 9.10.

* typo and lower case property name

Author: sublimemm

Guidelines.md

@@ -84,6 +84,7 @@ This document establishes the guidelines Microsoft REST APIs SHOULD follow so RE
         - [9.8.2. Client-driven paging](#982-client-driven-paging)
         - [9.8.3. Additional considerations](#983-additional-considerations)
     - [9.9. Compound collection operations](#99-compound-collection-operations)
+    - [9.10. Empty Results](#910-empty-results)
 - [10. Delta queries](#10-delta-queries)
     - [10.1. Delta links](#101-delta-links)
     - [10.2. Entity representation](#102-entity-representation)
@@ -1006,6 +1007,27 @@ When these operations are performed together, the evaluation order MUST be:
 2. **Sorting**. The potentially filtered list is sorted according to the sort criteria.
 3. **Pagination**. The materialized paginated view is presented over the filtered, sorted list. This applies to both server-driven pagination and client-driven pagination.
 
+### 9.10. Empty Results
+When a filter is performed on a collection and the result set is empty you MUST respond with a valid response body and a 200 response code. 
+In this example the filters supplied by the client resulted in a empty result set. 
+The response body is returned as normal and the _value_ attribute is set to a empty collection. 
+A client MAY be expecting metadata attributes like _maxItems_ based on the format of your responses to similar calls which produced results. 
+You SHOULD maintain consistency in your API whenever possible. 
+
+```http
+GET https://api.contoso.com/v1.0/products?$filter=(name eq 'Milk' or name eq 'Eggs') and price lt 2.55
+Accept: application/json
+
+HTTP/1.1 200 OK
+Content-Type: application/json
+
+{
+  ...,
+  "maxItems": 0,
+  "value": []
+}
+```
+
 ## 10. Delta queries
 Services MAY choose to support delta queries.