#415: fee() must validate that tab is non-empty

[VasilevNStas]

Problem

fee(tab, amount, summary, job) checks that tab is not nil but never checks for empty string — unlike every other string parameter in this library.

Solution

• Add raise(RuntimeError, "The \"tab\" may not be empty") if tab.empty? in BazaRb#fee
• Add the same check in BazaRb::Fake#fee
• Add test_fee_raises_when_tab_is_empty in both test files

Closes #415.

[VasilevNStas]

@yegor256 plz review this PR