KT-7756 Completion of "return null", "return true" and "return false"

#KT-7756 Fixed

idea/idea-completion/testData/handlers/keywords/ReturnNull.kt

@@ -0,0 +1,5 @@
+fun foo(): String? {
+    ret<caret>
+}
+
+// ELEMENT: "return null"

idea/idea-completion/testData/handlers/keywords/ReturnNull.kt.after

@@ -0,0 +1,5 @@
+fun foo(): String? {
+    return null<caret>
+}
+
+// ELEMENT: "return null"