fix: unbound variables in bash completion

when `set -o nounset` in Bash,
the warnings of unbound variables will break the bash completion.

use `kubectl` as example:

```sh
$ set -o nounset
$ my-cli <Tab>-bash: BASH_COMP_DEBUG_FILE: unbound variable
$
```

the warning break bash completion without any completion result,
and cause my cursor move to the newline.

Use `${variable:-}` substitution in Bash ,
that assign an empty string as default for unbound variables to fix the warnings.

bash_completions.go

@@ -24,7 +24,7 @@ func writePreamble(buf *bytes.Buffer, name string) {
 	buf.WriteString(fmt.Sprintf(`
 __%[1]s_debug()
 {
-    if [[ -n ${BASH_COMP_DEBUG_FILE} ]]; then
+    if [[ -n ${BASH_COMP_DEBUG_FILE:-} ]]; then
         echo "$*" >> "${BASH_COMP_DEBUG_FILE}"
     fi
 }
@@ -402,8 +402,8 @@ func writePostscript(buf *bytes.Buffer, name string) {
     local commands=("%[1]s")
     local must_have_one_flag=()
     local must_have_one_noun=()
-    local has_completion_function
-    local last_command
+    local has_completion_function=""
+    local last_command=""
     local nouns=()
 
     __%[1]s_handle_word