install(EXPORT): Fail earlier if EXPORT is missing

Add an explicit check in `install(EXPORT)` that the export name is
non-empty. Since an empty-named export set will never exist, this is
always an error. Previously, however, the error would not be caught
until generate time. Now an error will be produced immediately.

Source/cmInstallCommand.cxx

@@ -2076,6 +2076,11 @@ bool HandleExportMode(std::vector<std::string> const& args,
     return false;
   }
 
+  if (exp.empty()) {
+    status.SetError(cmStrCat(args[0], " missing EXPORT."));
+    return false;
+  }
+
   // Make sure there is a destination.
   if (ica.GetDestination().empty()) {
     // A destination is required.