main: Do not highlight empty region between two adjacent $()

Should fix zsh-users#624

Author: phy1729

highlighters/main/main-highlighter.zsh

@@ -1238,7 +1238,7 @@ _zsh_highlight_main_highlighter_highlight_double_quote()
   saved_reply=($reply)
   reply=()
   for 1 2 in $breaks; do
-    reply+=($1 $2 $style)
+    (( $1 != $2 )) && reply+=($1 $2 $style)
   done
   reply+=($saved_reply)
   REPLY=$i

highlighters/main/test-data/command-substitution-adjacent.zsh

@@ -0,0 +1,45 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2019 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+BUFFER='echo "$(echo)$(echo)'
+
+expected_region_highlight=(
+  '1 4 builtin' # echo
+  '6 20 default' # "$(echo)$(echo)
+  '6 6 double-quoted-argument-unclosed' # "
+  '7 13 command-substitution-quoted' # $(echo)
+  '7 8 command-substitution-delimiter-quoted' # $(
+  '9 12 builtin' # echo
+  '13 13 command-substitution-delimiter-quoted' # )
+  '14 20 command-substitution-quoted' # $(echo)
+  '14 15 command-substitution-delimiter-quoted' # $(
+  '16 19 builtin' # echo
+  '20 20 command-substitution-delimiter-quoted' # )
+)