Add test to show that jbuilder mishandles ${null}

Jbuilder incorrectly infers ${null} to be a target

test/blackbox-tests/jbuild

@@ -480,3 +480,12 @@
      (run ${exe:cram.exe} run.t)
      (diff? run.t run.t.corrected))))))
 
+(alias
+ ((name runtest)
+  (deps ((package jbuilder)
+         (files_recursively_in test-cases/null-dep)))
+  (action
+   (chdir test-cases/null-dep
+    (progn
+     (run ${exe:cram.exe} run.t)
+     (diff? run.t run.t.corrected))))))

test/blackbox-tests/test-cases/null-dep/jbuild

@@ -0,0 +1,5 @@
+(jbuild_version 1)
+
+(alias
+ ((name runtest)
+  (action (with-stdout-to ${null} (echo "hello world")))))

test/blackbox-tests/test-cases/null-dep/run.t

@@ -0,0 +1,4 @@
+  $ jbuilder runtest --debug-dependency-path
+  File "jbuild", line 5, characters 26-33:
+  Warning: Aliases must not have targets, this target will be ignored.
+  This will become an error in the future.