Bug fix

[Andriiuxa]

Fixes #
instead of having empty string in background-image, don't have background-image at all instead when there is not placeholderSrc passed

Description
background-image: "", makes a network call to this empty URL, instead it should just not be there in the first place if it was not specified

So at the moment we have to do something like this, to avoid a network call being made to ""

      <LazyLoadImage
        alt={imageAlt}
        src={`https://asapcdn.com/resize/?w=366&h=233&pad=true&i=${imageSrc}`}
        height="auto"
        style={{
          maxWidth: '100%',
          maxHeight: maxHeight || ''
        }}
        visibleByDefault={!lazyLoad}
        effect="blur"
        placeholderSrc={
          'data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw=='
        }
      />

https://imgur.com/a/E2O8zNQ - screenshot

cc - @Aljullu

[Aljullu]

Thanks for creating this PR @Andriiuxa! I added a small suggestion to make code more concise but this is overall looking good.

[Aljullu]

Thanks for applying the feedback @Andriiuxa, I merged your changes and released a new beta version.

Feel free to test it with npm i --save react-lazy-load-image-component@1.4.0beta.1 and report any issues you find.