Validate uploaded filename only if no upload error occured.

Most file upload errors (in particular UPLOAD_ERR_NO_FILE) yield an empty filename. The UploadFile validator does not report the original error in that case, but
FILE_NOT_FOUND instead.

This fix validates the filename only in case of UPLOAD_ERR_OK. Any other error code will be reported correctly.
Merge branch 'hotfix/6541'

Close #6541
[2.3.0] Link to new version of the QuickStart user guide
Fixes CS : trailing spaces and unused use
Merge pull request #6549 from michaelperrin/patch-1

[2.3.0] Link to new version of the QuickStart user guide
Merge branch 'hotfix/6549'

Close #6549
Merge branch 'hotfix/6549' into develop

Forward port #6549
Merge pull request #6551 from samsonasik/fix/cs-trailing-and-unused-use

Fixes CS : trailing spaces and unused use
Merge branch 'hotfix/6551'

Close #6551
Merge branch 'hotfix/6551' into develop

Forward port #6551
Merge pull request #6423 from hschletz/UploadErrorCodeShouldPrecedeEmptyFileCheck

Validate uploaded filename only if no upload error occured.
[#6423] No need to nest conditionals
Merge branch 'hotfix/6423' into develop

Forward port #6423