Do not set api version to language version when language version is null

#KT-21852 fixed
    #KT-21574 fixed

jps-plugin/testData/general/DefaultLanguageVersionCustomApiVersion/src/main.kt

@@ -0,0 +1,5 @@
+package test
+
+fun main(args: Array<String>) {
+    arrayListOf(1, 2, 3).fill(0)
+}