Fix NPE in SkipPatternSampler (#2089)

spring-cloud-sleuth-brave/src/main/java/org/springframework/cloud/sleuth/brave/instrument/web/SkipPatternSampler.java

@@ -34,6 +34,10 @@ abstract class SkipPatternSampler implements SamplerFunction<HttpRequest> {
 	@Override
 	public final Boolean trySample(HttpRequest request) {
 		String url = request.path();
+		if (url == null) {
+			return null;
+		}
+
 		boolean shouldSkip = pattern().matcher(url).matches();
 		if (shouldSkip) {
 			return false;

spring-cloud-sleuth-brave/src/test/java/org/springframework/cloud/sleuth/brave/instrument/web/SkipPatternSamplerTests.java

@@ -86,4 +86,16 @@ public class SkipPatternSamplerTests {
 		then(sampler.trySample(this.request)).isFalse();
 	}
 
+	@Test
+	public void should_delegate_sampling_decision_if_path_is_null() {
+		BDDMockito.given(this.request.path()).willReturn(null);
+		SkipPatternSampler sampler = new SkipPatternSampler() {
+			@Override
+			Pattern getPattern() {
+				return Pattern.compile("foo");
+			}
+		};
+
+		then(sampler.trySample(this.request)).isNull();
+	}
 }