Enforce UserImportOptions when importing users with passwords

[hiranya911]

When importing users with passwords, UserImportOptions must be specified. But the SDK does not check for this condition correctly. Instead it generates the following error when the user records contain passwords and UserImportOptions is not specified:

.../node_modules/firebase-admin/lib/auth/user-import-builder.js:176 if (!validator.isNonNullObject(options.hash)) { TypeError: Cannot read property 'hash' of undefined....

See https://stackoverflow.com/questions/56680087/firebase-admin-import-users-doesnt-work

[bojeil-google]

Just to confirm, is the issue that we are not validating options is nonNullObject first?

[hiranya911]

@bojeil-google. Yes, I think we should check options is specified before checking options.hash.