Issue #39 - Update ConditionInput to accept NULL value. Updated docs

Author: carolpettirossi

docs/search-parameters.md

@@ -103,7 +103,7 @@ evaluated against the search backend.
 |----------|----------|--------|-------------------------------------------------------------------------------------------------------------------------|
 | `operator` | no       | `String` | The operator allows us to specify the evaluation expression. All Search API conditions are supported. Some common examples: `=`, `<>`, `>`, `<`, `>=`, `<=`. |
 | `name`    | yes      | `String` | The name defines what field in the index to be evaluated against this condition.                                       |
-| `value`    | yes      | `String` | The value to be valuated against the specified field and operator.                                                      |                                    |
+| `value`    | no      | `String` | The value to be valuated against the specified field and operator. Leave it empty if you want to compare with NULL                                                      |                                    |
 
 ### Example
 Returns the all results with type of course.

src/Plugin/GraphQL/InputTypes/ConditionInput.php

@@ -12,7 +12,7 @@
  *   name = "ConditionInput",
  *   fields = {
  *     "name" = "String!",
- *     "value" = "String!",
+ *     "value" = "String",
  *     "operator" = "String"
  *   }
  * )