Replace nullFuzzedValue with primitive-aware defaultFuzzedValue (#2449)

* Replace `nullFuzzedValue` with primitive-aware `defaultFuzzedValue`

* Use `defaultFuzzedValue` only in `FieldValueProvider`

Author: IlyaMuravjov

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

@@ -39,10 +39,10 @@ class FieldValueProvider(
                 val thisInstanceValue = values.single()
                 val thisInstanceModel = when (val model = thisInstanceValue.model) {
                     is UtReferenceModel -> model
-                    is UtNullModel -> return@Create nullFuzzedValue(type.classId)
+                    is UtNullModel -> return@Create defaultFuzzedValue(type.classId)
                     else -> {
                         logger.warn { "This instance model can be only UtReferenceModel or UtNullModel, but $model is met" }
-                        return@Create nullFuzzedValue(type.classId)
+                        return@Create defaultFuzzedValue(type.classId)
                     }
                 }
                 UtAssembleModel(
@@ -62,7 +62,7 @@ class FieldValueProvider(
                 }
             },
             modify = emptySequence(),
-            empty = nullRoutine(type.classId)
+            empty = defaultValueRoutine(type.classId)
         )
     )
 }

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

@@ -2,6 +2,7 @@ package org.utbot.fuzzing.providers
 
 import org.utbot.framework.plugin.api.ClassId
 import org.utbot.framework.plugin.api.UtNullModel
+import org.utbot.framework.plugin.api.util.defaultValueModel
 import org.utbot.fuzzer.FuzzedType
 import org.utbot.fuzzer.FuzzedValue
 import org.utbot.fuzzer.fuzzed
@@ -11,4 +12,10 @@ fun nullRoutine(classId: ClassId): Routine.Empty<FuzzedType, FuzzedValue> =
     Routine.Empty { nullFuzzedValue(classId) }
 
 fun nullFuzzedValue(classId: ClassId): FuzzedValue =
-    UtNullModel(classId).fuzzed { summary = "%var% = null" }
+    UtNullModel(classId).fuzzed { summary = "%var% = null" }
+
+fun defaultValueRoutine(classId: ClassId): Routine.Empty<FuzzedType, FuzzedValue> =
+    Routine.Empty { defaultFuzzedValue(classId) }
+
+fun defaultFuzzedValue(classId: ClassId): FuzzedValue =
+    classId.defaultValueModel().fuzzed { summary = "%var% = $model" }