Provide quickfix for specifying type of variable initialized with null

#KT-23394 Fixed

idea/testData/quickfix/typeMismatch/variableInitializedWithNull.kt

@@ -0,0 +1,7 @@
+// "Change type of 'x' to 'String?'" "true"
+fun foo(condition: Boolean) {
+    var x = null
+    if (condition) {
+        x = "abc"<caret>
+    }
+}

idea/testData/quickfix/typeMismatch/variableInitializedWithNull.kt.after

@@ -0,0 +1,7 @@
+// "Change type of 'x' to 'String?'" "true"
+fun foo(condition: Boolean) {
+    var x: String? = null
+    if (condition) {
+        x = "abc"
+    }
+}

idea/testData/quickfix/typeMismatch/variableInitializedWithNull2.kt

@@ -0,0 +1,12 @@
+// "Change type of 'x' to 'String?'" "false"
+// ACTION: Remove braces from 'if' statement
+// ACTION: To raw string literal
+// ACTION: Convert assignment to assignment expression
+// ERROR: Type mismatch: inferred type is String but Nothing? was expected
+// ERROR: Val cannot be reassigned
+fun foo(condition: Boolean) {
+    val x = null
+    if (condition) {
+        x = "abc"<caret>
+    }
+}

idea/testData/quickfix/typeMismatch/variableInitializedWithNull3.kt

@@ -0,0 +1,11 @@
+// "Change type of 'x' to 'String?'" "false"
+// ACTION: Remove braces from 'if' statement
+// ACTION: To raw string literal
+// ACTION: Convert assignment to assignment expression
+// ERROR: Type mismatch: inferred type is String but Int? was expected
+fun foo(condition: Boolean) {
+    var x: Int? = null
+    if (condition) {
+        x = "abc"<caret>
+    }
+}

idea/testData/quickfix/typeMismatch/variableInitializedWithNull4.kt

@@ -0,0 +1,11 @@
+// "Change type of 'x' to 'String?'" "false"
+// ACTION: Remove braces from 'if' statement
+// ACTION: To raw string literal
+// ACTION: Convert assignment to assignment expression
+// ERROR: Type mismatch: inferred type is String but Int was expected
+fun foo(condition: Boolean) {
+    var x = 1
+    if (condition) {
+        x = "abc"<caret>
+    }
+}