core: (fixes #857): Handle default levels when log component is merely mentioned.

src/core/model/log.cc

@@ -181,11 +181,17 @@ LogComponent::EnvVarCheck()
         std::tie(found, value) = EnvironmentVariable::Get("NS_LOG", "***", ":");
     }
 
-    if (!found || value.empty())
+    if (!found)
     {
         return;
     }
 
+    if (value.empty())
+    {
+        // Default is enable all levels, all prefixes
+        value = "**";
+    }
+
     // Got a value, might have flags
     int level = 0;
     StringVector flags = SplitString(value, "|");