Blank spaces are not allowed when setting up string values for preference experiments

[SoftVision-CiprianMuresan]

[Affected versions]:

• Firefox Release 67.0.3, Build ID 20190618025334
• Firefox Beta 68.0b11, Build ID 20190617172838
• Firefox Nightly 69.0a1, Build ID 20190613215335

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Access to NORMANDY Delivery Console https://delivery-console.stage.mozaws.net/ is needed.

[Steps to reproduce]:

1. Start the browser and navigate to https://delivery-console.stage.mozaws.net.
2. Create a new pref experiment.
3. Select the Preference Type as string.
4. Add a branch and focus the Preference Value input field.
5. Press the space bar and observe the behavior.

[Expected results]:

• A blank space is inserted in the field.

[Actual results]:

• Nothing happens.

[Notes]:

• The issue is not reproducible when setting up preference rollouts.
• Blank spaces are accepted in the about:config page.
• Pasting a string that contains spaces works as expected.
• Attached a screen recording of the issue.