Fuzzer should create object without any modifications (#2319)

When fuzzer mutates objects, it should try to create an object without any modifications

Author: Markoutte

utbot-fuzzing/src/main/kotlin/org/utbot/fuzzing/Mutations.kt

@@ -303,7 +303,7 @@ sealed interface RecursiveMutations<TYPE, RESULT> : Mutation<Pair<Result.Recursi
         ): Result.Recursive<TYPE, RESULT> {
             return Result.Recursive(
                 construct = source.construct,
-                modify = source.modify.shuffled(random).take(random.nextInt(source.modify.size + 1).coerceAtLeast(1))
+                modify = source.modify.shuffled(random).take(random.nextInt(source.modify.size + 1))
             )
         }
     }

utbot-fuzzing/src/test/kotlin/org/utbot/fuzzing/FuzzerSmokeTest.kt

@@ -3,6 +3,7 @@ package org.utbot.fuzzing
 import kotlinx.coroutines.*
 import kotlinx.coroutines.flow.flow
 import org.junit.jupiter.api.Assertions
+import org.junit.jupiter.api.Assertions.fail
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
 import org.utbot.fuzzing.seeds.BitVectorValue
@@ -258,4 +259,27 @@ class FuzzerSmokeTest {
             deferred.cancel()
         }
     }
+
+    @Test
+    fun `fuzzer can generate value without mutations`() {
+        runBlocking {
+            var seenEmpty = false
+            withTimeout(1000) {
+                runFuzzing(
+                    { _, _ -> sequenceOf(Seed.Recursive<Unit, StringBuilder>(
+                        construct = Routine.Create(emptyList()) { StringBuilder("") },
+                        modify = sequenceOf(Routine.Call(emptyList()) { s, _ -> s.append("1") }),
+                        empty = Routine.Empty { fail("Empty is called despite construct requiring no args") }
+                    )) },
+                    Description(listOf(Unit))
+                ) { _, (s) ->
+                    if (s.isEmpty()) {
+                        seenEmpty = true
+                        BaseFeedback(Unit, Control.STOP)
+                    } else BaseFeedback(Unit, Control.CONTINUE)
+                }
+            }
+            Assertions.assertTrue(seenEmpty) { "Unmodified empty string wasn't generated" }
+        }
+    }
 }