Fix KNPE in "introduce variable", add some name conflict introduce tests

So #KT-21530 Fixed

idea/testData/refactoring/introduceProperty/kt21530.kt

@@ -0,0 +1,6 @@
+// EXTRACTION_TARGET: property with initializer
+
+fun test() = "123"
+
+val f = <selection>test()</selection> + "456"
+val x = test()

idea/testData/refactoring/introduceProperty/kt21530.kt.after

@@ -0,0 +1,8 @@
+// EXTRACTION_TARGET: property with initializer
+
+fun test() = "123"
+
+private val s = test()
+
+val f = s + "456"
+val x = s