Surround with null check: suggested for an assignment

#KT-30215 Fixed

idea/testData/quickfix/surroundWithNullCheck/assignment.kt

@@ -0,0 +1,6 @@
+// "Surround with null check" "true"
+
+fun foo(s: String?) {
+    var ss: String = ""
+    ss = <caret>s
+}

idea/testData/quickfix/surroundWithNullCheck/assignment.kt.after

@@ -0,0 +1,8 @@
+// "Surround with null check" "true"
+
+fun foo(s: String?) {
+    var ss: String = ""
+    if (s != null) {
+        ss = s
+    }
+}