update kotlin server samples

Author: wing328

samples/server/petstore/kotlin-springboot-reactive/pom.xml

@@ -96,6 +96,12 @@
             <artifactId>swagger-annotations</artifactId>
             <version>1.5.21</version>
         </dependency>
+        <!-- @Nullable annotation -->
+        <dependency>
+            <groupId>com.google.code.findbugs</groupId>
+            <artifactId>jsr305</artifactId>
+            <version>3.0.2</version>
+        </dependency>
         <dependency>
             <groupId>com.fasterxml.jackson.dataformat</groupId>
             <artifactId>jackson-dataformat-yaml</artifactId>

samples/server/petstore/kotlin-springboot/pom.xml

@@ -86,6 +86,12 @@
             <artifactId>swagger-annotations</artifactId>
             <version>1.5.21</version>
         </dependency>
+        <!-- @Nullable annotation -->
+        <dependency>
+            <groupId>com.google.code.findbugs</groupId>
+            <artifactId>jsr305</artifactId>
+            <version>3.0.2</version>
+        </dependency>
         <dependency>
             <groupId>com.fasterxml.jackson.dataformat</groupId>
             <artifactId>jackson-dataformat-yaml</artifactId>