Blur shader runs when menu blurriness is 0%

The blur shader is processed when the menu blurriness setting is set to 0%, causing the game to unnecessarily calculate a blur that has no effect. Relates to MC-268902, as users experience lag from running the shader even with 0% blurriness.

Attached screenshot shows blur lag caused by MC-268902 happening with 0% blurriness. Additionally, inspecting GameRenderer#processBlur(float) and calling code shows the blur shader is run regardless of blurriness setting.