Avoid NPE if @ConfigurationProperties is not present

The annotation processor detects `@ConfigurationProperties` bean or
method definition and merges manual meta-data. The former step will fail
with a NPE if the annotation is not present on the classpath. This could
happen if the annotation processor is added to a module that is not
actually using Spring Boot.

We now have a defensive check that skips that steps but still attempts to
merge manual meta-data if present.

Closes gh-3720

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

@@ -118,9 +118,12 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 			RoundEnvironment roundEnv) {
 		this.metadataCollector.processing(roundEnv);
 		Elements elementUtils = this.processingEnv.getElementUtils();
-		for (Element element : roundEnv.getElementsAnnotatedWith(elementUtils
-				.getTypeElement(configurationPropertiesAnnotation()))) {
-			processElement(element);
+		TypeElement annotationType = elementUtils
+				.getTypeElement(configurationPropertiesAnnotation());
+		if (annotationType != null) { // Is @ConfigurationProperties available
+			for (Element element : roundEnv.getElementsAnnotatedWith(annotationType)) {
+				processElement(element);
+			}
 		}
 		if (roundEnv.processingOver()) {
 			writeMetaData();