Optionally ignore null-valued derivation attributes

This allows adding attributes like

  attr = if stdenv.system == "bla" then something else null;

without changing the resulting derivation on non-<bla> platforms.

We once considered adding a special "ignore" value for this purpose,
but using null seems more elegant.

src/libexpr/eval.cc

@@ -139,6 +139,7 @@ EvalState::EvalState()
     , sSystem(symbols.create("system"))
     , sOverrides(symbols.create("__overrides"))
     , sOutputName(symbols.create("outputName"))
+    , sIgnoreNulls(symbols.create("__ignoreNulls"))
     , baseEnv(allocEnv(128))
     , baseEnvDispl(0)
     , staticBaseEnv(false, 0)