Changes related to stichoza/google-translate-php package

[Stichoza]

Changes proposed

• Bump version from ^5.0.1 to ^5.2.0.
  No backwards incompatible changes except ext-dom was added to requirements, which is bundled with PHP.
• Remove redundant check of $base_locale.
  The setSource() method already supports passing null as a parameter. So we can just use setSource($base_locale) even if the value is null (otherwise it will be string as defined in parameter type).
• Wider exception class
  The translate() might throw multiple different exceptions, most of them extend ErrorException, but some of them are extending UnexpectedValueException. Using Exception in catch block to cover all possible situations.
• Null coalescing operator
  The translate() method of my package might return null. Returning null will cause uncaught TypeError. Added null coalescing operator to handle this situation.

Other

• Returning false from method with return type string
  One thing that I didn't change is the return false in the catch block. Method has string as return type (as defined in ApiTranslatorContract). Returning false will not cause any errors like in case of null, but it will be converted to an empty string. I'm not sure if it's intended that way so I didn't touch it.