Fix incorrect error on read with 1-character input

When `read` is given a single character of input (including the
newline), it was bailing as if it had been given no input. This is
incorrect.

builtin.cpp

@@ -2655,7 +2655,7 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
             }
         }
 
-        if (buff.size() < 2 && eof)
+        if (buff.empty() && eof)
         {
             exit_res = 1;
         }

tests/read.in

@@ -38,6 +38,8 @@ echo 'test' | read -l one two three
 print_vars one two three
 echo 'foo   bar   baz' | read -l one two three
 print_vars one two three
+echo -n 'a' | read -l one
+echo "$status $one"
 
 echo
 set -l IFS

tests/read.out

@@ -17,6 +17,7 @@ two
 1 '' 1 ''
 1 'test' 1 '' 1 ''
 1 'foo' 1 'bar' 1 '  baz'
+0 a
 
 1 'hello'
 1 'h' 1 'ello'