Handle optionals with zero-length haystack

Match was bailing on haystack of zero length, without giving it a chance
to match against patterns which can, in fact, match nothing.

Author: mnemnion

README.md

@@ -11,7 +11,7 @@ The provided Regex type allows 64 'operations' and 8 unique ASCII character sets
 Drop the file into your project, or use the Zig build system:
 
 ```zig
-zig fetch --save "https://github.com/mnemnion/mvzr/archive/refs/tags/v0.3.1.tar.gz"
+zig fetch --save "https://github.com/mnemnion/mvzr/archive/refs/tags/v0.3.2.tar.gz"
 ```
 
 I'll do my best to keep that URL fresh, but it pays to check over here: ➔

build.zig.zon

@@ -7,7 +7,7 @@
 
     // This is a [Semantic Version](https://semver.org/).
     // In a future version of Zig it will be used for package deduplication.
-    .version = "0.3.1",
+    .version = "0.3.2",
 
     // This field is optional.
     // This is currently advisory only; Zig does not yet do anything

src/mvzr.zig

@@ -112,7 +112,6 @@ pub fn SizedRegex(ops: comptime_int, char_sets: comptime_int) type {
 
         /// Match a regex pattern in `haystack`, if found, this returns a `Match`.
         pub fn match(regex: *const SizedRegexT, haystack: []const u8) ?Match {
-            if (haystack.len == 0) return null;
             const maybe_matched = regex.matchInternal(haystack);
             if (maybe_matched) |m| {
                 const m1 = m[0];
@@ -2425,3 +2424,20 @@ test "Uppercase Greek" {
 test "M of N multibyte" {
     try testMatchEnd("abλ{3,5}", "abλλλλ");
 }
+
+test "zero length match on zero length haystack" {
+    const mvzr = @import("mvzr.zig");
+    const regex = mvzr.compile(".*");
+    const the_match = regex.?.match("");
+    try std.testing.expect(the_match != null);
+    try std.testing.expectEqual(0, the_match.?.start);
+    try std.testing.expectEqual(0, the_match.?.end);
+}
+test "zero length optional match on zero length haystack" {
+    const mvzr = @import("mvzr.zig");
+    const regex = mvzr.compile(".?");
+    const the_match = regex.?.match("");
+    try std.testing.expect(the_match != null);
+    try std.testing.expectEqual(0, the_match.?.start);
+    try std.testing.expectEqual(0, the_match.?.end);
+}