fix spammy log on empty GRPC_TRACE var
diff --git a/src/core/lib/debug/trace.cc b/src/core/lib/debug/trace.cc
index b0e0f2b..e6660c6 100644
--- a/src/core/lib/debug/trace.cc
+++ b/src/core/lib/debug/trace.cc
@@ -55,7 +55,7 @@
         found = true;
       }
     }
-    if (!found) {
+    if (!found && strcmp(name, "")) {
       gpr_log(GPR_ERROR, "Unknown trace var: '%s'", name);
       return false; /* early return */
     }