Add defensive check in TestMain to ensure environment variables are always set

Add check to ensure sharedContainer is not nil before setting environment
variables. This helps catch issues where container creation succeeds but
returns nil.

Author: zph

mysql/testcontainers_testmain.go

@@ -73,9 +73,16 @@ func TestMain(m *testing.M) {
 	}
 
 	// Set up environment variables for the shared container
-	os.Setenv("MYSQL_ENDPOINT", sharedContainer.Endpoint)
-	os.Setenv("MYSQL_USERNAME", sharedContainer.Username)
-	os.Setenv("MYSQL_PASSWORD", sharedContainer.Password)
+	// These MUST be set even if sharedContainer is nil (shouldn't happen, but be defensive)
+	if sharedContainer != nil {
+		os.Setenv("MYSQL_ENDPOINT", sharedContainer.Endpoint)
+		os.Setenv("MYSQL_USERNAME", sharedContainer.Username)
+		os.Setenv("MYSQL_PASSWORD", sharedContainer.Password)
+	} else {
+		// This should never happen, but if it does, fail loudly
+		os.Stderr.WriteString(fmt.Sprintf("ERROR: startSharedMySQLContainer returned nil container without error for image '%s'\n", dockerImage))
+		os.Exit(1)
+	}
 
 	// Run all tests
 	code := m.Run()