completions: bash: add missing functions

Add empty functions for all commands defined in the autocompletion script.
When these functions are not defined, bash raises the following error:

       $ meson init <TAB>-bash: _meson-init: command not found

Signed-off-by: Liam Beguin <liambeguin@gmail.com>

data/shell-completions/bash/meson

@@ -242,6 +242,10 @@ _meson-configure() {
   fi
 }
 
+_meson-install() {
+  : TODO
+}
+
 _meson-introspect() {
   shortopts=(
     h
@@ -286,6 +290,10 @@ _meson-introspect() {
   fi
 }
 
+_meson-init() {
+  : TODO
+}
+
 _meson-test() {
   shortopts=(
     q
@@ -414,3 +422,11 @@ for test in json.load(sys.stdin):
 _meson-wrap() {
   : TODO
 }
+
+_meson-subprojects() {
+  : TODO
+}
+
+_meson-help() {
+  : # Nothing to do
+}