refactor: add isDefiningProjection helper re: #5881

Author: vkarpov15

lib/services/projection/isDefiningProjection.js

@@ -0,0 +1,18 @@
+'use strict';
+
+/*!
+ * ignore
+ */
+
+module.exports = function isDefiningProjection(val) {
+  if (val == null) {
+    // `undefined` or `null` become exclusive projections
+    return true;
+  }
+  if (typeof val === 'object') {
+    // Only cases where a value does **not** define whether the whole projection
+    // is inclusive or exclusive are `$meta` and `$slice`.
+    return !('$meta' in val) && !('$slice' in val);
+  }
+  return true;
+}