Update Task Thin Executions list function for parity with 2.11.x

Updated task thin executions link handing and added extra test for parity with 2.11.x.
Update minimum supported version of client.

See spring-attic#6062

Author: Corneil du Plessis

spring-cloud-dataflow-rest-client/src/main/java/org/springframework/cloud/dataflow/rest/client/TaskTemplate.java

@@ -142,7 +142,12 @@ public class TaskTemplate implements TaskOperations {
 		this.restTemplate = restTemplate;
 
 		String version = VersionUtils.getThreePartVersion(dataFlowServerVersion);
-		Assert.isTrue(VersionUtils.isDataFlowServerVersionGreaterThanOrEqualToRequiredVersion(version, VALIDATION_MIN_VERSION), () -> "Minimum Data Flow version required is " + VALIDATION_MIN_VERSION + " but got " + version);
+		if (StringUtils.hasText(version)) {
+			Assert.isTrue(
+					VersionUtils.isDataFlowServerVersionGreaterThanOrEqualToRequiredVersion(version,
+							VALIDATION_MIN_VERSION),
+					() -> "Minimum Data Flow version required is " + VALIDATION_MIN_VERSION + " but got " + version);
+		}
 
 		this.aboutLink = resources.getLink("about").get();