[string] Match everything with an empty pattern and -e

I.e. `string match --entire "" -- banana` used to match nothing, now
it matches everything.

Fixes #4971.

src/builtin_string.cpp

@@ -628,7 +628,8 @@ class wildcard_matcher_t : public string_matcher_t {
                 wcpattern[i] = towlower(wcpattern[i]);
             }
         }
-        if (opts.entire && !wcpattern.empty()) {
+        if (opts.entire) {
+            // If the pattern is empty, this becomes one ANY_STRING that matches everything.
             if (wcpattern.front() != ANY_STRING) wcpattern.insert(0, 1, ANY_STRING);
             if (wcpattern.back() != ANY_STRING) wcpattern.push_back(ANY_STRING);
         }

tests/string.err

@@ -283,6 +283,9 @@ string repeat -l fakearg
 ####################
 # string match --entire -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abcx caabxyxz
 
+####################
+# string match --entire "" -- banana
+
 ####################
 # string match -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abcx caabxyxz
 

tests/string.in

@@ -289,6 +289,10 @@ logmsg 'string match --entire -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abc
 string match --entire -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abcx caabxyxz
 or echo exit 1
 
+logmsg 'string match --entire "" -- banana'
+string match --entire "" -- banana
+or echo exit 1
+
 logmsg 'string match -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abcx caabxyxz'
 string match -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abcx caabxyxz
 or echo exit 1

tests/string.out

@@ -384,6 +384,10 @@ kaabxz
 abbxy
 caabxyxz
 
+####################
+# string match --entire "" -- banana
+banana
+
 ####################
 # string match -r "a*b[xy]+" abc abxc bye aaabyz kaabxz abbxy abcx caabxyxz
 abx