WASM: Properly handle nullable exported strings

libraries/stdlib/wasm/internal/kotlin/wasm/internal/ImportExport.kt

@@ -8,7 +8,10 @@ package kotlin.wasm.internal
 // This is called when exported function returns a string. It writes [i32 length, [i16 chars ...]] into a temporary raw memory area and
 // returns pointer to the start of it.
 // Note: currently there is a single temporary raw memory area so it's not possible to export more than one string at a time.
-internal fun exportString(src: String): Int {
+internal fun exportString(src: String?): Int {
+    if (src == null)
+        throw IllegalArgumentException("Exporting null string")
+
     val retAddr = unsafeGetScratchRawMemory(INT_SIZE_BYTES + src.length * CHAR_SIZE_BYTES)
     wasm_i32_store(retAddr, src.length)
     unsafeCharArrayToRawMemory(src.toCharArray(), retAddr + INT_SIZE_BYTES)