Problem with custom skybox when there is no cubemap

I'm using a cubemap with stars (one layer per channel) as a mask over the skybox, tinting both skybox and stars with a separate color.
The shader works fine when there is a cubemap reference but when there is none the cubemap reads a 0.5 grey affecting the skybox color.
I've already tried to set its default value to black with no luck.
Any ideas on how to approach this issue?

Here's a simplified version of the shader:

Code (CSharp):

1. Shader "Example"
2. {
3.     Properties
4.     {
5.         _StarsCubemap ("Stars Cubemap", Cube) = "black" {}
6.         _StarsColor("Stars Color", Color) = (1, 1, 1, 1)
7.         _SkyboxColor("Skybox Color", Color) = (0, 0, 0, 0)
8.     }
9.     SubShader
10.     {
11.         Tags
12.         {
13.             "PreviewType" = "Skybox"
14.             "Queue" = "Background"
15.             "RenderType" = "Background"
16.         }
17.  
18.         Pass
19.         {
20.             CGPROGRAM
21.             #pragma vertex vert
22.             #pragma fragment frag
23.             #include "UnityCG.cginc"
24.  
25.             struct appdata
26.             {
27.                 float4 uv0 : TEXCOORD0;
28.                 float4 vertex : POSITION;
29.             };
30.  
31.             struct v2f
32.             {
33.                 float4 uv0 : TEXCOORD0;
34.                 float4 vertex : SV_POSITION;
35.             };
36.  
37.             samplerCUBE _StarsCubemap;
38.             half4 _StarsColor;
39.             half4 _SkyboxColor;
40.  
41.             v2f vert(appdata v)
42.             {
43.                 v2f o;
44.                 o.vertex = UnityObjectToClipPos(v.vertex);
45.                 o.uv0 = v.uv0;
46.                 return o;
47.             }
48.  
49.             fixed4 frag(v2f i) : SV_Target
50.             {
51.                 half4 starsMask = texCUBE(_StarsCubemap, i.uv0.xyz);
52.                 half4 finalColor = lerp(_SkyboxColor, _StarsColor, starsMask.r); // Using red channel as a mask
53.                 return finalColor;
54.             }
55.             ENDCG
56.         }
57.     }
58. }
59.  

 

Unfortunately I believe the only default cubemap textures Unity provides as a default is that middle grey texture.

https://docs.unity3d.com/Manual/SL-Properties.html

There are two main options to solve this. One is provide your own all black cubemap texture. The other is to have a material property you use to switch the cubemap off.