Issue-11: Fix return type of Client::runQuery and ::runRawQuery to return Results instead of ?Results

Author: mghoneimy

src/Client.php

@@ -47,10 +47,10 @@ public function __construct(string $endpointUrl, array $authorizationHeaders = [
  * @param bool $resultsAsArray
  * @param array $variables
  *
- * @return Results|null
+ * @return Results
  * @throws QueryError
  */
- public function runQuery($query, bool $resultsAsArray = false, array $variables = []): ?Results
+ public function runQuery($query, bool $resultsAsArray = false, array $variables = []): Results
  {
  if ($query instanceof QueryBuilderInterface) {
  $query = $query->getQuery();
@@ -68,10 +68,10 @@ public function runQuery($query, bool $resultsAsArray = false, array $variables
  * @param bool $resultsAsArray
  * @param array $variables
  *
- * @return Results|null
+ * @return Results
  * @throws QueryError
  */
- public function runRawQuery(string $queryString, $resultsAsArray = false, array $variables = []): ?Results
+ public function runRawQuery(string $queryString, $resultsAsArray = false, array $variables = []): Results
  {
  // Set request headers for authorization and content type
  if (!empty($this->authorizationHeaders)) {