Replace some String.length() checks with String.isEmpty()

Closes gh-10451

spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootConfigurationFinder.java

@@ -63,7 +63,7 @@ final class SpringBootConfigurationFinder {
 	}
 
 	private Class<?> scanPackage(String source) {
-		while (source.length() > 0) {
+		while (!source.isEmpty()) {
 			Set<BeanDefinition> components = this.scanner.findCandidateComponents(source);
 			if (!components.isEmpty()) {
 				Assert.state(components.size() == 1,

spring-boot-test/src/main/java/org/springframework/boot/test/web/client/MockServerRestTemplateCustomizer.java

@@ -98,7 +98,7 @@ public class MockServerRestTemplateCustomizer implements RestTemplateCustomizer
 	}
 
 	public MockRestServiceServer getServer() {
-		Assert.state(this.servers.size() > 0,
+		Assert.state(!this.servers.isEmpty(),
 				"Unable to return a single MockRestServiceServer since "
 						+ "MockServerRestTemplateCustomizer has not been bound to "
 						+ "a RestTemplate");