Revert "std.fmt.format: small optimization to avoid runtime bloat"

This reverts commit 11d38a7e52.

The benefits of this commit are not enough to justify the compromise
that it made.

closes #5977

lib/std/fmt.zig

@@ -88,8 +88,6 @@ pub fn format(
     if (args.len > ArgSetType.bit_count) {
         @compileError("32 arguments max are supported per format call");
     }
-    if (args.len == 0)
-        return writer.writeAll(fmt);
 
     const State = enum {
         Start,