initialise 'customTags' parameter with default value

[evidolob]

What does this PR do?

It fixes error:

Error - 2:38:37 pm] (node:5928) UnhandledPromiseRejectionWarning: TypeError: customTags.filter is not a function 
at Object.filterInvalidCustomTags (c:\Users\_username_\.vscode\extensions\redhat.vscode-yaml-0.20.0\node_modules\yaml-language-server\out\server\src\languageservice\utils\arrUtils.js:56:23)
at Object.customTagsToAdditionalOptions (c:\Users\_username_\.vscode\extensions\redhat.vscode-yaml-0.20.0\node_modules\yaml-language-server\out\server\src\languageservice\utils\parseUtils.js:51:37) 
at Object.parse (c:\Users\_username_\.vscode\extensions\redhat.vscode-yaml-0.20.0\node_modules\yaml-language-server\out\server\src\languageservice\parser\yamlParser07.js:79:44) 
at YamlDocuments.ensureCache (c:\Users\_username_\.vscode\extensions\redhat.vscode-yaml-0.20.0\node_modules\yaml-language-server\out\server\src\languageservice\parser\yaml-documents.js:44:40)
at YamlDocuments.getYamlDocument (c:\Users\_username_\.vscode\extensions\redhat.vscode-yaml-0.20.0\node_modules\yaml-language-server\

Find by telemetry

[coveralls]

Coverage increased (+0.005%) to 78.473% when pulling 8cf6bb3 on evidolob:fix-customtag-filter into 9008ead on redhat-developer:main.