(core) Fix two issues combining to report misleading error when saving to an empty name

Summary:
1. The /import endpoint wasn't handling poor names like ".grist" as
   intended, instead trying to import them using the plugin-based imports.

2. The SaveCopy dialog was allowing users to save to an empty name,
   particularly bad because new docs now default to an empty name.

Error manifested as "Cannot parse data" to the user.
Reported in https://secure.helpscout.net/conversation/1242629116/292

Test Plan: Added tests for both parts of the fix.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2573

app/server/lib/DocManager.ts

@@ -417,12 +417,12 @@ export class DocManager extends EventEmitter {
                              }): Promise<DocCreationInfo> {
     try {
       const fileCount = uploadInfo.files.length;
-      const hasGristDoc = Boolean(uploadInfo.files.find(f => path.extname(f.origName) === '.grist'));
+      const hasGristDoc = Boolean(uploadInfo.files.find(f => extname(f.origName) === '.grist'));
       if (hasGristDoc && fileCount > 1) {
         throw new Error('Grist docs must be uploaded individually');
       }
       const first = uploadInfo.files[0].origName;
-      const ext = path.extname(first);
+      const ext = extname(first);
       const basename = path.basename(first, ext).trim() || "Untitled upload";
       let id: string;
       switch (options.naming) {
@@ -479,3 +479,9 @@ export class DocManager extends EventEmitter {
     return docName;
   }
 }
+
+// Returns the extension of fpath (from last occurrence of "." to the end of the string), even
+// when the basename is empty or starts with a period.
+function extname(fpath: string): string {
+  return path.extname("X" + fpath);
+}