fix: on error empty null source [#1028]

[mistryswapnil-dream11]

The PR fixes the following DylanVann#1028.

Previously it was observed that onError was not getting invoked when an empty or null source uri was passed to FastImage as a prop.
The PR fixes point mentioned above.

Scenarios Tested:

1. When source uri is valid and not reachable, onError should be invoked:

    import React from 'react'
    import {View} from 'react-native'
    import FastImage from 'react-native-fast-image'

    export const ImageLoadErrorExample = () => {

    const onImageLoadError = () => {
        console.log("Error called when url is not reachable.")
    }

    return <View>
        <FastImage source={{
            uri: 'https://fastly.picsum.photos/id/31'
        }}
        onError={onImageLoadError}
        />
    </View>
}

2. When source uri is empty (""), onError should be invoked:

    import React from 'react'
    import {View} from 'react-native'
    import FastImage from 'react-native-fast-image'

    export const ImageLoadErrorExample = () => {

    const onImageLoadError = () => {
        console.log("Error called when url is empty.")
    }

    return <View>
        <FastImage source={{
            uri: ''
        }}
        onError={onImageLoadError}
        />
    </View>
}

3. When source uri is null, onError should be invoked:

import React from 'react'
   import {View} from 'react-native'
   import FastImage from 'react-native-fast-image'

   export const ImageLoadErrorExample = () => {

   const onImageLoadError = () => {
       console.log("Error called when url is null.")
   }

   return <View>
       <FastImage source={{
           uri: null
       }}
       onError={onImageLoadError}
       />
   </View>
}

4. When source uri is a fallback source, is valid URL, but not reachable:

import React from 'react'
import {View} from 'react-native'
import FastImage from 'react-native-fast-image'

export const ImageLoadErrorExample = () => {

const onImageLoadError = () => {
    console.log("Error called when fallback url is not reachable.")
}

return <View>
    <FastImage source={{
        uri: 'https://fastly.picsum.photos/id/31'
    }}
    onError={onImageLoadError}
    fallback={true}
    />
</View>
}