Mempool WAL is still created by default in home directory, leads to permission errors (#2758)

* only invoke InitWAL/CloseWAL if WalPath is not empty

Closes #2717

* panic if WAL is not initialized when calling CloseWAL

* add a changelog entry

config/config.go

@@ -497,6 +497,11 @@ func (cfg *MempoolConfig) WalDir() string {
 	return rootify(cfg.WalPath, cfg.RootDir)
 }
 
+// WalEnabled returns true if the WAL is enabled.
+func (cfg *MempoolConfig) WalEnabled() bool {
+	return cfg.WalPath != ""
+}
+
 // ValidateBasic performs basic validation (checking param bounds, etc.) and
 // returns an error if any check fails.
 func (cfg *MempoolConfig) ValidateBasic() error {