Avoid deprecation warnings in newer PHP

[mrsdizzie]

To avoid deprecation warnings in PHP 8.1+:

PHP Deprecated:  str_replace(): Passing null to parameter #3 ($subject)
of type array|string is deprecated in
/Users/isla/source/wp-cli-dev/php-cli-tools/lib/cli/table/Tabular.php on
line 30

I saw this while running tests for wp-cli/package-command#197 where some rows of wp package browse look like:

        array(4) {
          [0]=>
          string(22) "redkiwi-nl/localconfig"
          [1]=>
          NULL
          [2]=>
          string(0) ""
          [3]=>
          string(54) "0.1, 0.2.1, 0.2.2, 0.3, 0.3.1, 9999999-dev, dev-master"
        }
        array(4) {
          [0]=>
          string(18) "vccw/scaffold-vccw"
          [1]=>
          NULL
          [2]=>
          string(0) ""
          [3]=>
          string(84) "1.0.0, 1.0.1, 1.1.0, 1.1.1, 1.2.0, 1.3.0, 1.4.0, 9999999-dev, dev-master, dev-stable"
        }
        array(4) {
          [0]=>
          string(22) "welaika/wp-cli-db2utf8"
          [1]=>
          NULL
          [2]=>
          string(0) ""
          [3]=>
          string(23) "9999999-dev, dev-master"
        }

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[swissspidy]

Good find, thanks! Appreciate the added test, as the existing tests didn't cover this.