`rbenv-version-name` returns `system` for empty version files. Closes #62, #65, #77.

libexec/rbenv-version-file-read

@@ -13,6 +13,6 @@ if [ -e "$VERSION_FILE" ]; then
       break
     fi
   done < <( cat "$VERSION_FILE" && echo )
-else
-  exit 1
 fi
+
+exit 1

libexec/rbenv-version-name

@@ -6,8 +6,8 @@ if [ -z "$RBENV_VERSION" ]; then
   RBENV_VERSION="$(rbenv-version-file-read "$RBENV_VERSION_FILE" || true)"
 fi
 
-if [ "$RBENV_VERSION" = "system" ]; then
-  echo "$RBENV_VERSION"
+if [ -z "$RBENV_VERSION" ] || [ "$RBENV_VERSION" = "system" ]; then
+  echo "system"
   exit
 fi