chore(e2e): fix for enos docker build (#6166)

* chore(e2e): fix for enos docker build

enos/enos-variables.hcl

@@ -275,7 +275,7 @@ variable "windows_instance_type" {
 variable "ui_build_override" {
   description = "Override for build for UI automation"
   type        = string
-  default     = null
+  default     = ""
 }
 
 variable "github_token" {

enos/modules/build_boundary_docker_local/main.tf

@@ -26,7 +26,8 @@ variable "edition" {
 
 variable "ui_build_override" {
   description = "Override for build for UI automation (oss or ent)"
-  default     = null
+  type        = string
+  default     = ""
 }
 
 resource "enos_local_exec" "get_git_sha" {

enos/modules/build_local/main.tf

@@ -35,7 +35,8 @@ variable "goos" {
 
 variable "ui_build_override" {
   description = "Override for build for UI automation (oss or ent)"
-  default     = null
+  type        = string
+  default     = ""
 }
 
 resource "enos_local_exec" "build" {