wallet2: don't try to lock an empty filename

src/wallet/wallet2.cpp

@@ -7555,6 +7555,8 @@ bool wallet2::is_output_blackballed(const std::pair<uint64_t, uint64_t> &output)
 
 bool wallet2::lock_keys_file()
 {
+  if (m_wallet_file.empty())
+    return true;
   if (m_keys_file_locker)
   {
     MDEBUG(m_keys_file << " is already locked.");
@@ -7566,6 +7568,8 @@ bool wallet2::lock_keys_file()
 
 bool wallet2::unlock_keys_file()
 {
+  if (m_wallet_file.empty())
+    return true;
   if (!m_keys_file_locker)
   {
     MDEBUG(m_keys_file << " is already unlocked.");
@@ -7577,6 +7581,8 @@ bool wallet2::unlock_keys_file()
 
 bool wallet2::is_keys_file_locked() const
 {
+  if (m_wallet_file.empty())
+    return false;
   return m_keys_file_locker->locked();
 }