Skip leading empty ranges in roundRobin

Previously, roundRobin would always attempt to fetch its first element
from the first range passed to it, even if that range was empty.

Fixes bugzilla issue 24384.

std/range/package.d

@@ -2427,7 +2427,14 @@ if (Rs.length > 1 && allSatisfy!(isInputRange, staticMap!(Unqual, Rs)))
         }
     }
 
-    return Result(rs, 0);
+    size_t firstNonEmpty = size_t.max;
+    static foreach (i; 0 .. Rs.length)
+    {
+        if (firstNonEmpty == size_t.max && !rs[i].empty)
+            firstNonEmpty = i;
+    }
+
+    return Result(rs, firstNonEmpty);
 }
 
 ///
@@ -2489,6 +2496,14 @@ pure @safe nothrow unittest
     assert(equal(r, [ S(1), S(10), S(2), S(20) ]));
 }
 
+// https://issues.dlang.org/show_bug.cgi?id=24384
+@safe unittest
+{
+    auto r = roundRobin("", "a");
+    assert(!r.empty);
+    auto e = r.front;
+}
+
 /**
 Iterates a random-access range starting from a given point and
 progressively extending left and right from that point. If no initial