It can be done with Views rewriting options.

1. Add both fields (Image before Video) to Views and exclude Image field from output.
   
2. For Video field set “NO RESULTS BEHAVIOR” to Image field placeholder:

It means that if Video field is empty Views will get value from this textarea, which contains special placeholders (in this case [field_image]).

Placeholders for your field can be found under “REWRITE RESULTS” fieldset:

It can be done with Views rewriting options.

1. Add both fields (Image before Video) to Views and exclude Image field from output.
   
2. For Video field set “NO RESULTS BEHAVIOR” to Image field token:

It means that if Video field is empty Views will get value from this textarea, which contains special placeholders (in this case [field_image]).

Available tokens for your fields can be found under “REWRITE RESULTS” fieldset:

Don't forget to uncheck “Rewrite the output of this field” flag.