Don't warn about clearing empty query

[mrkazoodle]

Is this a bug report?

(write your answer here)
No: an enhancement.

Environment

1. java -version: openjdk 1.8.0_151
2. node -v (If you use Node.js proxy for Saiku):
3. npm -v (If you use Node.js proxy for Saiku):
4. yarn --version (If you use Node.js proxy for Saiku and if you use Yarn):

Then, specify:

1. Operating system: Ubuntu 16.04 LTS
   Browser and version (if relevant): Firefox 58.0.2 (64-bits)
2. Saiku plugin to Pentaho or Saiku Standalone: Standalone
3. Saiku CE or Saiku EE: CE
4. Saiku version: 3.15

Steps to Reproduce

(Write your steps here:)

1. Open a query
2. Select a cube
3. Select a different cube

Expected Behavior

(Write what you thought would happen.)
A different cube is selected.

Actual Behavior

(Write what happened. Please add screenshots!)
A warning is given "new query - you are about to clear your existing query". Only, after clicking "okay", a different cube is selected.

This warning has no use: nothing is to be lost if the query is empty. The request only required a check which should be easy to implement.

[buggtb]

Feel free to submit a patch! :)