Put default value (null or zero) to slot for uninitialized values

This way range of the variable is correct in LVT.
 #KT-24672 Fixed

compiler/testData/codegen/bytecodeText/localInitializationLVT/contract.kt

@@ -0,0 +1,25 @@
+// IGNORE_BACKEND: JVM_IR
+
+import kotlin.contracts.ExperimentalContracts
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
+@ExperimentalContracts
+fun test(): Char {
+    val c: Char
+    doIt {
+        c = ' '
+    }
+    return c
+}
+
+@ExperimentalContracts
+fun doIt(block: () -> Unit) {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    block()
+}
+
+// 2 FCONST 0
+// 1 LOCALVARIABLE c Lkotlin/jvm/internal/Ref$CharRef; L1 L10 0