Fixes cases of invalid image folder path and input column name

[mstfbl]

Fixes #4429

Input column names and image folders can no longer be empty or null.

[antoniovs1029]

Hi, @mstfbl

About the exception thrown if the inputFolder doesn't exist, that's generally ok. If anything, I left some comments there.

About the other changes in ImageAnalytics/ExtensionsCatalog.cs they're not ok. I am afraid I didn't explain myself clearly on issue #4429 , so I am thinking about updating that issue in order to make it more clear. The issue isn't that inputColumnName can be null, the problem I described there is that the inputColumn can be empty for all rows (i.e. that content of the column, for every row, is empty) and the trainer would still train and classify every row.

So I would revert the changes you made in that file, since the behavior it had for them was actually all right (i.e. if inputColumnName is null then it should default to outputColumnName, as this is done throughout our API).

[antoniovs1029]

Also, it seems that all of the pipelines are failing because of the changes on the imageFolder parameter. It seems it's supposed to be an optional parameter, so it should actually support being "null". In that case I guess you should also remove the changes you made to that, and only check if inputFolder exists if it wasn't "null" to begin with.

[mstfbl]

Thanks @antoniovs1029 for your review. I made new changes after reviewing your input. I'll make a new PR to implement the other changes once you clarify your issue.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d85be83). Click here to learn what that means.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master    #4691   +/-   ##
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  Coverage          ?   75.83%           
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  Files             ?      951           
  Lines             ?   172384           
  Branches          ?    18613           
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  Hits              ?   130734           
  Misses            ?    36474           
  Partials          ?     5176

Flag	Coverage Δ
#Debug	75.83% <66.66%> (?)
#production	71.42% <66.66%> (?)
#test	90.62% <ø> (?)

Impacted Files	Coverage Δ
src/Microsoft.ML.ImageAnalytics/ImageLoader.cs	85.11% <66.66%> (ø)

[antoniovs1029]

Thanks @antoniovs1029 for your review. I made new changes after reviewing your input. I'll make a new PR to implement the other changes once you clarify your issue.

Hi, I had added my clarifications on the issue. Idk if it's clear enough, though, let me know if you need further explanations. Thanks!