Handle null support policy values from Contentful

Fixes gh-18

src/main/java/io/spring/projectapi/contentful/ContentfulQueries.java

@@ -17,6 +17,7 @@
 package io.spring.projectapi.contentful;
 
 import java.time.Duration;
+import java.util.Collections;
 import java.util.List;
 import java.util.function.Function;
 
@@ -76,7 +77,13 @@ private ClientGraphQlResponse executeForSingleProject(String documentName, Strin
 	}
 
 	private <T> List<T> fieldToEntityList(ClientGraphQlResponse response, String path, Class<T> type) {
-		return field(response, path, (field) -> field.toEntityList(type));
+		return field(response, path, (field) -> getList(type, response, path));
+	}
+
+	private static <T> List<T> getList(Class<T> type, ClientGraphQlResponse response, String path) {
+		ClientResponseField field = response.field(path);
+		return (!field.hasValue() && response.getErrors().isEmpty()) ? Collections.emptyList()
+				: field.toEntityList(type);
 	}
 
 	private <T> T fieldToEntity(ClientGraphQlResponse response, String path, Class<T> type) {

src/test/java/io/spring/projectapi/contentful/ContentfulQueriesTests.java

@@ -133,6 +133,13 @@ void getProjectSupportsWhenNoProjectMatchThrowsException() throws IOException {
 				.satisfies((ex) -> assertThat(ex.getProjectSlug()).isEqualTo("spring-xd"));
 	}
 
+	@Test
+	void getProjectSupportsWhenNullReturnsEmptyList() throws IOException {
+		setupResponse("query-null-support.json");
+		List<ProjectSupport> supports = this.contentfulQueries.getProjectSupports("spring-xd");
+		assertThat(supports).isEmpty();
+	}
+
 	private void setupResponse(String name) throws IOException {
 		setupResponse(new ClassPathResource(name, getClass()));
 	}

src/test/resources/io/spring/projectapi/contentful/query-error.json

@@ -1,4 +1,15 @@
 {
   "data": {
-  }
+  },
+  "errors": [
+    {
+      "message": "Cannot query field \"a\" on type \"Project\".",
+      "locations": [
+        {
+          "line": 4,
+          "column": 4
+        }
+      ]
+    }
+  ]
 }

src/test/resources/io/spring/projectapi/contentful/query-null-support.json

@@ -0,0 +1,11 @@
+{
+  "data": {
+    "projectCollection": {
+      "items": [
+        {
+          "support": null
+        }
+      ]
+    }
+  }
+}