lib.attrsets.hasAttrByPath: Document law and laziness, and test it

lib/attrsets.nix

@@ -51,12 +51,19 @@ rec {
 
   /* Return if an attribute from nested attribute set exists.
 
+     **Laws**:
+      1.  ```nix
+          hasAttrByPath [] x == true
+          ```
+
      Example:
        x = { a = { b = 3; }; }
        hasAttrByPath ["a" "b"] x
        => true
        hasAttrByPath ["z" "z"] x
        => false
+       hasAttrByPath [] (throw "no need")
+       => true
 
     Type:
       hasAttrByPath :: [String] -> AttrSet -> Bool

lib/tests/misc.nix

@@ -697,6 +697,11 @@ runTests {
     expected = false;
   };
 
+  testHasAttrByPathNonStrict = {
+    expr = hasAttrByPath [] (throw "do not use");
+    expected = true;
+  };
+
   testLongestValidPathPrefix_empty_empty = {
     expr = attrsets.longestValidPathPrefix [ ] { };
     expected = [ ];