fix spammy log on empty GRPC_TRACE var

src/core/lib/debug/trace.cc

@@ -55,7 +55,7 @@ bool TraceFlagList::Set(const char* name, bool enabled) {
         found = true;
       }
     }
-    if (!found) {
+    if (!found && strcmp(name, "")) {
       gpr_log(GPR_ERROR, "Unknown trace var: '%s'", name);
       return false; /* early return */
     }