cli: make rg -vf file behave sensibly

Previously, when `file` is empty (literally empty, as in, zero byte),
`rg -f file` and `rg -vf file` would behave identically. This is odd
and also doesn't match how GNU grep behaves. It's also not logically
correct. An empty file means _zero_ patterns which is an empty set. An
empty set matches nothing. Inverting the empty set should result in
matching everything.

This was because of an errant optimization that lets ripgrep quit early
if it can statically detect that no matches are possible.

Moreover, there was *also* a bug in how we constructed the PCRE2 pattern
when there are zero patterns. PCRE2 doesn't have a concept of sets of
patterns (unlike the `regex` crate), so we need to fake it with an empty
character class.

Fixes #1332, Fixes #3001, Closes #3041

crates/core/flags/hiargs.rs

@@ -517,7 +517,7 @@ impl HiArgs {
     /// When this returns false, it is impossible for ripgrep to ever report
     /// a match.
     pub(crate) fn matches_possible(&self) -> bool {
-        if self.patterns.patterns.is_empty() {
+        if self.patterns.patterns.is_empty() && !self.invert_match {
             return false;
         }
         if self.max_count == Some(0) {

crates/pcre2/src/matcher.rs

@@ -55,7 +55,12 @@ impl RegexMatcherBuilder {
                 format!("(?:{})", p.as_ref())
             });
         }
-        let mut singlepat = pats.join("|");
+        let mut singlepat = if patterns.is_empty() {
+            // A way to spell a pattern that can never match anything.
+            r"[^\S\s]".to_string()
+        } else {
+            pats.join("|")
+        };
         if self.case_smart && !has_uppercase_literal(&singlepat) {
             builder.caseless(true);
         }