Sema: correct OPV for optional empty error set

prevents crashes in backends; improves codegen; provides more
comptime-ness.

test/cases/compile_errors/optional_empty_error_set.zig

@@ -0,0 +1,13 @@
+export fn example() void {
+    comptime foo() catch |err| switch (err) {};
+}
+var x: ?error{} = null;
+fn foo() !void {
+    return x.?;
+}
+// error
+// backend=stage2
+// target=native
+//
+// :6:13: error: unable to unwrap null
+// :2:17: note: called at comptime here

test/cases/safety/optional_empty_error_set.zig

@@ -0,0 +1,22 @@
+const std = @import("std");
+
+pub fn panic(message: []const u8, stack_trace: ?*std.builtin.StackTrace, ra: ?usize) noreturn {
+    _ = stack_trace;
+    _ = ra;
+    if (std.mem.eql(u8, message, "attempt to use null value")) {
+        std.process.exit(0);
+    }
+    std.process.exit(1);
+}
+
+pub fn main() !void {
+    foo() catch |err| switch (err) {};
+    return error.TestFailed;
+}
+var x: ?error{} = null;
+fn foo() !void {
+    return x.?;
+}
+// run
+// backend=stage2,llvm
+// target=native