[LyX/master] Amend 28be7d55 to avoid an exception

[Enrico Forestieri]

commit 7b5d33a70746f42364b7d8ff944e7a768b508e95
Author: Enrico Forestieri <for...@lyx.org>
Date:   Tue Jun 27 00:09:51 2017 +0200

    Amend 28be7d55 to avoid an exception
    
    This was occurring during validation when trying to enter the
    language name in the advanced tab.
---
 src/insets/InsetListingsParams.cpp |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/insets/InsetListingsParams.cpp 
b/src/insets/InsetListingsParams.cpp
index 0daf408..503200d 100644
--- a/src/insets/InsetListingsParams.cpp
+++ b/src/insets/InsetListingsParams.cpp
@@ -1050,8 +1050,8 @@ void InsetListingsParams::addParam(string const & key,
        if (key.empty())
                return;
 
-       string const value = (minted() && key == "language") ? languageonly(val)
-                                                            : val;
+       string const value = (minted() && key == "language" && !val.empty())
+                               ? languageonly(val) : val;
        // duplicate parameters!
        string keyname = key;
        if (!replace && hasParam(key))