default query that affects entire library is dangerous

Maybe this is subjective, but I don't think so... Basically, the default query being '' is extremely dangerous. A simple typo on the command line can result in something quite bad such as:

beet modify genre="Electro"

Which is the same as:

beet modify genre="Electro" ""

It should take a lot more deliberateness to do a library-wide change like that. And in fact, something like "beet info" does the right thing in that "beet info" does nothing and you have to issue:

beet info ' '

To get "all". Which, by the way, is a second argument for making this change. Probably we want consistency across the various command line arguments as much as possible.

Related, arguably, the query for all should be "*", but maybe that's a whole separate ticket.

Start by reading the command-line handling for `beet modify` and `beet info`, especially how an empty query is interpreted. Compare their current behavior with the proposed deliberate all-library query, including whether `*` is in scope; done when library-wide changes require explicit input and related commands are consistent.

Written by the indexing model from the issue text.