Enable an empty \substack (#2278)

test/katex-spec.js

@@ -2660,6 +2660,9 @@ describe("A substack function", function() {
     it("should accommodate macros in the argument", function() {
         expect`\sum_{\substack{ 0<i<\varPi \\ 0<j<\pi }}  P(i,j)`.toBuild();
     });
+    it("should accommodate an empty argument", function() {
+        expect`\sum_{\substack{}}  P(i,j)`.toBuild();
+    });
 
 });