lib: fix gcc warning in printf call

Do not pass NULL to printf %s.

Seen with gcc 13.2.0 on Debian:
```
.../curl/lib/connect.c:696:27: warning: '%s' directive argument is null [-Wformat-overflow=]
```
Ref: https://github.com/curl/curl-for-win/actions/runs/6476161689/job/17584426483#step:3:11104

Ref: curl#10284
Co-authored-by: Jay Satiro
Closes curl#12082

lib/connect.c

@@ -693,11 +693,11 @@ static CURLcode is_connected(struct Curl_cfilter *cf,
   result = CURLE_COULDNT_CONNECT;
   for(i = 0; i < sizeof(ctx->baller)/sizeof(ctx->baller[0]); i++) {
     struct eyeballer *baller = ctx->baller[i];
+    if(!baller)
+      continue;
     CURL_TRC_CF(data, cf, "%s assess started=%d, result=%d",
-                baller?baller->name:NULL,
-                baller?baller->has_started:0,
-                baller?baller->result:0);
-    if(baller && baller->has_started && baller->result) {
+                baller->name, baller->has_started, baller->result);
+    if(baller->has_started && baller->result) {
       result = baller->result;
       break;
     }