Make Spring Java fuzzer fallback to null value when nothing else works (#2448)

* Make Java fuzzer fallback to `null` value when nothing else works

* Use `AnyDepthNullValueProvider` only in Spring integration tests

Author: IlyaMuravjov

utbot-framework/src/main/kotlin/org/utbot/framework/context/spring/SpringIntegrationTestConcreteExecutionContext.kt

@@ -15,6 +15,7 @@ import org.utbot.framework.plugin.api.util.utContext
 import org.utbot.fuzzer.IdentityPreservingIdGenerator
 import org.utbot.fuzzing.JavaValueProvider
 import org.utbot.fuzzing.ValueProvider
+import org.utbot.fuzzing.providers.AnyDepthNullValueProvider
 import org.utbot.fuzzing.providers.FieldValueProvider
 import org.utbot.fuzzing.providers.ObjectValueProvider
 import org.utbot.fuzzing.providers.anyObjectValueProvider
@@ -93,6 +94,7 @@ class SpringIntegrationTestConcreteExecutionContext(
             .with(springBeanValueProvider)
             .with(createSavedEntityValueProviders(relevantRepositories, idGenerator))
             .with(createFieldValueProviders(relevantRepositories, idGenerator))
+            .withFallback(AnyDepthNullValueProvider)
     }
 
     private fun createSavedEntityValueProviders(

utbot-java-fuzzing/src/main/kotlin/org/utbot/fuzzing/providers/Objects.kt

@@ -132,6 +132,21 @@ object NullValueProvider : ValueProvider<FuzzedType, FuzzedValue, FuzzedDescript
     }
 }
 
+/**
+ * Unlike [NullValueProvider] can generate `null` values at any depth.
+ *
+ * Intended to be used as a last fallback.
+ */
+object AnyDepthNullValueProvider : ValueProvider<FuzzedType, FuzzedValue, FuzzedDescription> {
+
+    override fun accept(type: FuzzedType) = type.classId.isRefType
+
+    override fun generate(
+        description: FuzzedDescription,
+        type: FuzzedType
+    ) = sequenceOf<Seed<FuzzedType, FuzzedValue>>(Seed.Simple(nullFuzzedValue(classClassId)))
+}
+
 /**
  * Finds and create object from implementations of abstract classes or interfaces.
  */