SDKtool: Do not add empty CMake configuration values

Do not add the empty string as CMake configuration. People are
apparently trying to use the empty string to make sure nothing
is set up.

Task-number: QTCREATORBUG-21696
Change-Id: I702e26bc135e30c36f770210f885a4680d4b0359
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/tools/sdktool/addkitoperation.cpp

@@ -260,7 +260,8 @@ bool AddKitOperation::setArguments(const QStringList &args)
             if (next.isNull())
                 return false;
             ++i;
-            m_cmakeConfiguration.append(next);
+            if (!next.isEmpty())
+                m_cmakeConfiguration.append(next);
             continue;
         }