assignment followed by expansion of null/unset word fails assertion

[grisenti]

command:

a=a $b

output:

mrsh: ../shell/task/simple_command.c:286: run_simple_command: Assertion `args.len > 0' failed.
fish: Job 1, './buildDir/mrsh' terminated by signal SIGABRT (Abort)

I think the issue is here, which checks if there is a word that isn't an assignment, but this isn't correct because the word could expand to nothing.