inconsistent =~ behavior in LogQL with empty matchers

[sandeepsukhani]

Describe the bug
The behavior of =~ with empty matches("") behaves inconsistently (oppositely) depending on whether it is being used in a stream selector or a line filter expression. When it is used in index labels, it excludes all the streams with a label with the given name, while as a line filter, it matches everything.

For example:
The following query matches nothing when the probe label always has values:
{service_name="foo", probe=~""} | logfmt

Same query, except moving the stream filter out of the {} and using it as a line filter expression:
{service_name="foo"} | logfmt | probe=~""

This query returns everything that matches {service_name="brand-new-check"}, regardless of the probe value.

Code-wise, we need to change the behaviour in regex simpler. However, it might be hard to fix this issue since it would be a breaking change because, suddenly, users would stop seeing data for queries which earlier returned non-empty responses.