[LyX/master] Fix wrong warning.

[Juergen Spitzmueller]

commit da4e51a6c2987ef03d5f618f9cc43c87a2992fa4
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Mon May 21 13:45:31 2018 +0200

    Fix wrong warning.
---
 src/TextClass.cpp |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/src/TextClass.cpp b/src/TextClass.cpp
index 4788fef..f33dc18 100644
--- a/src/TextClass.cpp
+++ b/src/TextClass.cpp
@@ -1769,6 +1769,9 @@ DocumentClassPtr getDocumentClass(
                }
        }
 
+       if (cengine.empty())
+               return doc_class;
+
        LyXCiteEngine * ce = theCiteEnginesList[cengine];
        if (!ce) {
                docstring const msg =