Implement GL_EXT_null_initializer

Adds null initializer syntax (empty braces)
Allows null initialization of shared variables

Test/vulkan.comp

@@ -7,6 +7,27 @@ layout(local_size_z_id = 14) in;  // ERROR, can't change this
 void main()
 {
     gl_WorkGroupSize;
+    int i = { };  // ERROR, need an extension
 }
 
 layout(local_size_y_id = 19) in; // ERROR, already used: TODO not yet reported
+
+shared float f = { };  // ERROR, need an extension
+float g = { };         // ERROR, need an extension
+
+#extension GL_EXT_null_initializer : enable
+
+shared float f2 = { };
+float g2 = { };
+
+void foo()
+{
+    int i = { };
+    float fa[] = { };
+}
+
+struct samp {
+    sampler2D s2D;
+} sampVar = { };
+
+atomic_uint a = { };