I was under the impression that は should be always used in negative sentences. Is がない an exception to this rule? Does が add something special to the negation?
Do the above sentences have the same meaning?
I think this が is が for neutral description.
neutral description (ga) only works with action verbs, existential verbs, and adjectives/nominal adjectives that represent state change. "Sentences of neutral description present an objectively observable action, existence, or temporary state as a new event."
This type of が is not limited to がない.
- あの看板がよく見えない。 I can't see that signboard well.
- あれ？ パソコンが動かない。 Oh? This PC doesn't work.
When you say 仕事はない, you are saying it more or less as the plain fact that holds true for the time being (e.g., you are unemployed). 仕事がない is used when you have realized you have nothing to do for now, or you are temporarily having trouble finding a good job.
It might be useful to picture them as being answers to different “questions”:
Q1. (Is there a job? / Are there any jobs?)
Q2. (What do you not have? / Why are you broke? / Why are you at home?)
You would never answer Q1 with A2, and vice versa.