ledger-mode: Tab-completion of account name only works when the account has an explicit amount

[Ekleog]

Example:

2000-01-01 * foo
    abc  1
    def

2000-01-01 * bar
    d<tab> //< doesn't complete in def, while a<tab> would complete in abc

Thank you anyway for this great standard! :)

[tbm]

@enderw88 Craig can you look at this?