overwrite 'msbuild_toolset' only if 'toolset' is defined (#1242)

node-gyp uses 'msbuild_toolset' in 'target_defaults' to specify what
toolset should be used, according to what version of Visual Studio
was detected.

Before this change, when 'toolset' was not specified, node-sqlite3
failed to compile with any version of Visual Studio after 2015,
because gyp fails to detect the toolset correctly.

With this change 'toolset' is only used if defined, fixing
compilation with node-gyp and recent VS versions.

deps/common-sqlite.gypi

@@ -5,7 +5,11 @@
   },
   'target_defaults': {
     'default_configuration': 'Release',
-    'msbuild_toolset':'<(toolset)',
+    'conditions': [
+      [ 'toolset!=""', {
+        'msbuild_toolset':'<(toolset)'
+      }]
+    ],
     'configurations': {
       'Debug': {
         'defines!': [