Enable an empty \substack (#2278)

src/environments/array.js

@@ -737,7 +737,7 @@ defineEnvironment({
             arraystretch: 0.5,
         };
         res = parseArray(context.parser, res, "script");
-        if (res.body[0].length > 1) {
+        if (res.body.length > 0 &&  res.body[0].length > 1) {
             throw new ParseError("{subarray} can contain only one column");
         }
         return res;

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();
+    });
 
 });