[Hotfix] Validator\File classes behaviour with empty value

[stefanotorresi]

an empty $_FILES array would be passed to file validators under some circumstances, e.g. trying to upload a file that exceeds the php.ini upload_max_filesize value.

stream_resolve_include_path() is used in place of file_exists() across the whole Validator\File package and, while being faster, it actually returns the current working directory when called with an empty argument.

this PR prevents the subsequent inconsistent behaviour of all the validators using stream_resolve_include_path() when trying to validate an empty value. (triggering errors/warnings, displaying wrong failed validation message, etc)

Done/To-do

• Crc32
• ExcludeExtension
• ExcludeMimeType
• Extension
• FileSize
• Hash
• ImageSize
• Md5
• MimeType
• Size
• UploadFile
• WordCount