Fix a crash when opening an empty file.

Fixes: https://github.com/solvespace/solvespace/issues/918

The problem was that here:
11a8a0abd5/src/file.cpp (L480)
we cleared all groups. Then we tried to read an empty file and therefore this `while` exited immediately:
11a8a0abd5/src/file.cpp (L487)
and we came to here:
11a8a0abd5/src/file.cpp (L548)
where there was no `fileLoadError` and thus we were left with no groups, and so this assert failed:
11a8a0abd5/src/graphicswin.cpp (L394)
since is is not allowed to have no groups :-)

src/file.cpp

@@ -468,6 +468,7 @@ void SolveSpaceUI::LoadUsingTable(const Platform::Path &filename, char *key, cha
 }
 
 bool SolveSpaceUI::LoadFromFile(const Platform::Path &filename, bool canCancel) {
+    bool fileIsEmpty = true;
     allConsistent = false;
     fileLoadError = false;
 
@@ -485,6 +486,8 @@ bool SolveSpaceUI::LoadFromFile(const Platform::Path &filename, bool canCancel)
 
     char line[1024];
     while(fgets(line, (int)sizeof(line), fh)) {
+        fileIsEmpty = false;
+
         char *s = strchr(line, '\n');
         if(s) *s = '\0';
         // We should never get files with \r characters in them, but mailers
@@ -545,6 +548,11 @@ bool SolveSpaceUI::LoadFromFile(const Platform::Path &filename, bool canCancel)
 
     fclose(fh);
 
+    if(fileIsEmpty) {
+        Error(_("The file is empty. It may be corrupt."));
+        NewFile();
+    }
+
     if(fileLoadError) {
         Error(_("Unrecognized data in file. This file may be corrupt, or "
                 "from a newer version of the program."));