GeneratorSequence: do not get first value, until it's requested by iterator, allow initialValue to be null and return EmptySequence in this case.

#KT-9153 Fixed

libraries/stdlib/test/collections/SequenceTest.kt

@@ -1,6 +1,7 @@
 package test.collections
 
 import org.junit.Test as test
+import org.junit.Test
 import kotlin.test.*
 import java.util.*
 
@@ -233,6 +234,24 @@ public class SequenceTest {
         assertEquals(expected, values.toList(), "Iterating sequence second time yields the same result")
     }
 
+    @Test fun sequenceFromFunctionWithLazyInitialValue() {
+        var start = 3
+        val values = sequence({ start }, { n -> if (n > 0) n - 1 else null })
+        val expected = listOf(3, 2, 1, 0)
+        assertEquals(expected, values.toList())
+        assertEquals(expected, values.toList(), "Iterating sequence second time yields the same result")
+
+        start = 2
+        assertEquals(expected.drop(1), values.toList(), "Initial value function is called on each iterator request")
+
+        // does not throw on construction
+        val errorValues = sequence<Int>({ (throw IllegalStateException()) }, { null })
+        // does not throw on iteration
+        val iterator = errorValues.iterator()
+        // throws on advancing
+        assertFails { iterator.next() }
+    }
+
 
     @test fun sequenceFromIterator() {
         val list = listOf(3, 2, 1, 0)