Fix NoClassDefFoundError when Mockito is missing

Update MockReset class to check for the presence of the MockUtil class
before attempting to use it.

Fixes gh-7065

spring-boot-samples/spring-boot-sample-test-nomockito/src/main/java/sample/testnomockito/SampleTestNoMockitoApplication.java

@@ -0,0 +1,13 @@
+package sample.testnomockito;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class SampleTestNoMockitoApplication {
+
+	public static void main(String[] args) {
+		SpringApplication.run(SampleTestNoMockitoApplication.class);
+	}
+
+}