How to find variable is empty in shell script

(1).

var=""
if [ -n "$var" ]; then
    echo "not empty"
else
    echo "empty"
fi

(2).

function empty
{
    local var="$1"

# Return true if:
    # 1.    var is a null string ("" as empty string)
    # 2.    a non set variable is passed
    # 3.    a declared variable or array but without a value is passed
    # 4.    an empty array is passed
    if test -z "$var"
    then
        [[ $( echo "1" ) ]]
        return

# Return true if var is zero (0 as an integer or "0" as a string)
    elif [ "$var" == 0 2> /dev/null ]
    then
        [[ $( echo "1" ) ]]
        return

# Return true if var is 0.0 (0 as a float)
    elif [ "$var" == 0.0 2> /dev/null ]
    then
        [[ $( echo "1" ) ]]
        return
    fi

[[ $( echo "" ) ]]
}
usage:
if empty "${var}"
    then
        echo "empty"
    else
        echo "not empty"
fi

(3).

#!/bin/bash

vars=(
    ""
    0
    0.0
    "0"
    1
    "string"
    " "
)

for (( i=0; i<${#vars[@]}; i++ ))
do
    var="${vars[$i]}"

if empty "${var}"
        then
            what="empty"
        else
            what="not empty"
    fi
    echo "VAR \"$var\" is $what"
done

exit
output:
VAR "" is empty
VAR "0" is empty
VAR "0.0" is empty
VAR "0" is empty
VAR "1" is not empty
VAR "string" is not empty
VAR " " is not empty