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

idea/testData/refactoring/introduceVariable/kt21530_withConstructorParam.kt

@@ -0,0 +1,8 @@
+fun test() = "123"
+
+open class First(val s: String)
+object Second : First(<selection>test()</selection> + "456")
+
+fun foo(test: String) {
+    val x = test()
+}

idea/testData/refactoring/introduceVariable/kt21530_withConstructorParam.kt.after

@@ -0,0 +1,11 @@
+fun test() = "123"
+
+open class First(val s: String)
+
+val test1 = test()
+
+object Second : First(test1 + "456")
+
+fun foo(test: String) {
+    val x = test1
+}

idea/testData/refactoring/introduceVariable/kt21530_withGlobal.kt

@@ -0,0 +1,4 @@
+fun test() = "123"
+
+val f = <selection>test()</selection> + "456"
+val x = test()

idea/testData/refactoring/introduceVariable/kt21530_withGlobal.kt.after

@@ -0,0 +1,5 @@
+fun test() = "123"
+
+val test = test()
+val f = test + "456"
+val x = test

idea/testData/refactoring/introduceVariable/kt21530_withParam.kt

@@ -0,0 +1,7 @@
+fun test() = "123"
+
+val f = <selection>test()</selection> + "456"
+
+fun foo(test: String) {
+    val x = test()
+}

idea/testData/refactoring/introduceVariable/kt21530_withParam.kt.after

@@ -0,0 +1,8 @@
+fun test() = "123"
+
+val test1 = test()
+val f = test1 + "456"
+
+fun foo(test: String) {
+    val x = test1
+}