Avoid writing empty stage variable

Avoid writing the stage variable if an empty string is passed in

ee/lib/security/ci_configuration/sast_build_actions.rb

@@ -36,7 +36,7 @@ module Security
       end
 
       def sast_stage
-        @params['stage'] || 'test'
+        @params['stage'].presence ? @params['stage'] : 'test'
       end
 
       # We only want to write variables that are set

ee/spec/lib/security/ci_configuration/sast_build_actions_spec.rb

@@ -6,8 +6,12 @@ RSpec.describe Security::CiConfiguration::SastBuildActions do
   context 'autodevops disabled' do
     let(:auto_devops_enabled) { false }
 
-    context 'with one empty parameter' do
-      let(:params) { { 'SECURE_ANALYZERS_PREFIX' => '' } }
+    context 'with empty parameters' do
+      let(:params) do
+        { 'stage' => '',
+          'SECURE_ANALYZERS_PREFIX' => '',
+          'SEARCH_MAX_DEPTH' => '' }
+      end
 
       subject(:result) { described_class.new(auto_devops_enabled, params).generate }