When you say "there is nobody in the garden" is the answer would be (are they? ) I just have read before that in ques. Tag we use (they) when the question include (everybody/nobody..etc
A sentence that begins with There is is called an existential clause, the sentence refers to the existence or presence of something.
In English, existential clauses usually use the dummy subject construction (also known as expletive) with there, as in "There are boys in the yard"…
In the OP's sentence, the subject is not "nobody" but there (is). Consequently, the rule dictates that you should repeat the same subject used in the clause to make a question tag.
There is nobody in the garden, is there?
As illustrated by FumbleFinger's comment, when the main clause begins with the subject "nobody", or "everybody", "everyone", "no one", etc. THEY is used in the question tag.
Nobody is in the garden, are they?
The pronoun nobody makes the clause negative, so the verb used in question tag should be positive. However, if the pronoun everybody/everyone is used, the question tag should be in the negative.
Everybody is in the garden, aren't they?