bash-completion: make completions to work when bash set -u is in use

User who want to avoid refering to none-existing variables got earlier the
following error.

$ set -u
$ findmnt --output <tab>bash: OUTPUT: unbound variable

Here is short explanation of this setting.

$ help set
 -u  Treat unset variables as an error when substituting.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

bash-completion/blkid

@@ -59,7 +59,7 @@ _blkid_module()
 		prefix="${cur%$realcur}"
 		for WORD in $OUTPUT_ALL; do
 			if ! [[ $prefix == *"$WORD"* ]]; then
-				OUTPUT="$WORD $OUTPUT"
+				OUTPUT="$WORD ${OUTPUT:-""}"
 			fi
 		done
 		compopt -o nospace

bash-completion/fdisk

@@ -49,7 +49,7 @@ _fdisk_module()
 			"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/fincore

@@ -12,7 +12,7 @@ _fincore_module()
 			OUTPUT_ALL='PAGES SIZE FILE RES'
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/findmnt

@@ -55,7 +55,7 @@ _findmnt_module()
 
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/losetup

@@ -33,7 +33,7 @@ _losetup_module()
 				SIZELIMIT DIO"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/lsipc

@@ -35,7 +35,7 @@ _lsipc_module()
 			"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/lslocks

@@ -20,7 +20,7 @@ _lslocks_module()
 			OUTPUT_ALL="COMMAND PID TYPE SIZE MODE M START END PATH BLOCKER"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/lsmem

@@ -12,7 +12,7 @@ _lsmem_module()
 			OUTPUT_ALL='RANGE SIZE STATE REMOVABLE BLOCK NODE'
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/partx

@@ -15,7 +15,7 @@ _partx_module()
 			prefix="${cur%$realcur}"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/prlimit

@@ -17,7 +17,7 @@ _prlimit_module()
 			OUTPUT_ALL="DESCRIPTION RESOURCE SOFT HARD UNITS"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/sfdisk

@@ -28,7 +28,7 @@ _sfdisk_module()
 			"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/swapon

@@ -18,7 +18,7 @@ _swapon_module()
 			OUTPUT_ALL="NAME TYPE SIZE USED PRIO UUID LABEL"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/wdctl

@@ -29,7 +29,7 @@ _wdctl_module()
 			OUTPUT_ALL="FLAG DESCRIPTION STATUS BOOT-STATUS DEVICE"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace

bash-completion/zramctl

@@ -18,7 +18,7 @@ _zramctl_module()
 				MIGRATED MOUNTPOINT"
 			for WORD in $OUTPUT_ALL; do
 				if ! [[ $prefix == *"$WORD"* ]]; then
-					OUTPUT="$WORD $OUTPUT"
+					OUTPUT="$WORD ${OUTPUT:-""}"
 				fi
 			done
 			compopt -o nospace