Ensure that SignatureInformation has a non-null array

While the LSP allows a SignatureInformation's parameters array to be
optional, Monaco does not so we should ensure that it is initialized
with a zero length array if the value from the language server is null
or undefined.

Signed-off-by: Remy Suen <remy.suen@gmail.com>

src/monaco-converter.ts

@@ -613,7 +613,11 @@ export class ProtocolToMonacoConverter {
     asSignatureInformation(item: SignatureInformation): monaco.languages.SignatureInformation {
         let result = <monaco.languages.SignatureInformation>{ label: item.label };
         if (item.documentation) { result.documentation = this.asDocumentation(item.documentation); }
-        if (item.parameters) { result.parameters = this.asParameterInformations(item.parameters); }
+        if (item.parameters) {
+            result.parameters = this.asParameterInformations(item.parameters);
+        } else {
+            result.parameters = [];
+        }
         return result;
     }