Fixed potentially uninitialized value warnings emitted by clang

Removed unused variable from DigitalInput class

Removed extraneous explicit std::string conversions in Preferences class

Change-Id: Ia445abfd136a7b3e7f0491ed22aaa057814bcc8f

wpilibc/wpilibC++Devices/src/Preferences.cpp

@@ -336,9 +336,9 @@ std::string Preferences::Get(const char *key) {
   Synchronized sync(m_tableLock);
   if (key == nullptr) {
     wpi_setWPIErrorWithContext(NullParameter, "key");
-    return std::string("");
+    return "";
   }
-  return m_values[std::string(key)];
+  return m_values[key];
 }
 
 /**