fix: unbound variables in bash completion (spf13#1321)

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 io.StringWriter, name string) {
 	WriteStringAndCheck(buf, fmt.Sprintf(`
 __%[1]s_debug()
 {
-    if [[ -n ${BASH_COMP_DEBUG_FILE} ]]; then
+    if [[ -n ${BASH_COMP_DEBUG_FILE:-} ]]; then
         echo "$*" >> "${BASH_COMP_DEBUG_FILE}"
     fi
 }
@@ -187,7 +187,7 @@ __%[1]s_handle_reply()
                     PREFIX=""
                     cur="${cur#*=}"
                     ${flags_completion[${index}]}
-                    if [ -n "${ZSH_VERSION}" ]; then
+                    if [ -n "${ZSH_VERSION:-}" ]; then
                         # zsh completion needs --flag= prefix
                         eval "COMPREPLY=( \"\${COMPREPLY[@]/#/${flag}=}\" )"
                     fi
@@ -278,7 +278,7 @@ __%[1]s_handle_flag()
 
     # if a command required a flag, and we found it, unset must_have_one_flag()
     local flagname=${words[c]}
-    local flagvalue
+    local flagvalue=""
     # if the word contained an =
     if [[ ${words[c]} == *"="* ]]; then
         flagvalue=${flagname#*=} # take in as flagvalue after the =
@@ -297,7 +297,7 @@ __%[1]s_handle_flag()
 
     # keep flag value with flagname as flaghash
     # flaghash variable is an associative array which is only supported in bash > 3.
-    if [[ -z "${BASH_VERSION}" || "${BASH_VERSINFO[0]}" -gt 3 ]]; then
+    if [[ -z "${BASH_VERSION:-}" || "${BASH_VERSINFO[0]:-}" -gt 3 ]]; then
         if [ -n "${flagvalue}" ] ; then
             flaghash[${flagname}]=${flagvalue}
         elif [ -n "${words[ $((c+1)) ]}" ] ; then
@@ -369,7 +369,7 @@ __%[1]s_handle_word()
         __%[1]s_handle_command
     elif __%[1]s_contains_word "${words[c]}" "${command_aliases[@]}"; then
         # aliashash variable is an associative array which is only supported in bash > 3.
-        if [[ -z "${BASH_VERSION}" || "${BASH_VERSINFO[0]}" -gt 3 ]]; then
+        if [[ -z "${BASH_VERSION:-}" || "${BASH_VERSINFO[0]:-}" -gt 3 ]]; then
             words[c]=${aliashash[${words[c]}]}
             __%[1]s_handle_command
         else
@@ -410,8 +410,8 @@ func writePostscript(buf io.StringWriter, name string) {
     local command_aliases=()
     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=()
     local noun_aliases=()
 
@@ -621,7 +621,7 @@ func writeCmdAliases(buf io.StringWriter, cmd *Command) {
 
 	sort.Strings(cmd.Aliases)
 
-	WriteStringAndCheck(buf, fmt.Sprint(`    if [[ -z "${BASH_VERSION}" || "${BASH_VERSINFO[0]}" -gt 3 ]]; then`, "\n"))
+	WriteStringAndCheck(buf, fmt.Sprint(`    if [[ -z "${BASH_VERSION:-}" || "${BASH_VERSINFO[0]:-}" -gt 3 ]]; then`, "\n"))
 	for _, value := range cmd.Aliases {
 		WriteStringAndCheck(buf, fmt.Sprintf("        command_aliases+=(%q)\n", value))
 		WriteStringAndCheck(buf, fmt.Sprintf("        aliashash[%q]=%q\n", value, cmd.Name()))