fix: unbound variables in bash completion

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

for example:

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

Author: edentsai

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 ${BASH_COMP_DEBUG_FILE:-} ]]; then
         echo "$*" >> "${BASH_COMP_DEBUG_FILE}"
     fi
 }
@@ -214,7 +214,7 @@ __%[1]s_handle_reply()
     completions=("${commands[@]}")
     if [[ ${#must_have_one_noun[@]} -ne 0 ]]; then
         completions+=("${must_have_one_noun[@]}")
-    elif [[ -n "${has_completion_function}" ]]; then
+    elif [[ -n "${has_completion_function:-}" ]]; then
         # if a go completion function is provided, defer to that function
         __%[1]s_handle_go_custom_completion
     fi
@@ -334,7 +334,7 @@ __%[1]s_handle_command()
     __%[1]s_debug "${FUNCNAME[0]}: c is $c words[c] is ${words[c]}"
 
     local next_command
-    if [[ -n ${last_command} ]]; then
+    if [[ -n ${last_command:-} ]]; then
         next_command="_${last_command}_${words[c]//:/__}"
     else
         if [[ $c -eq 0 ]]; then