Allow using an empty string with pipeline expressions

lib/gitlab/ci/pipeline/expression/lexeme/string.rb

@@ -4,7 +4,7 @@ module Gitlab
       module Expression
         module Lexeme
           class String < Lexeme::Value
-            PATTERN = /("(?<string>.+?)")|('(?<string>.+?)')/.freeze
+            PATTERN = /("(?<string>.*?)")|('(?<string>.*?)')/.freeze
 
             def initialize(value)
               @value = value

spec/lib/gitlab/ci/pipeline/expression/lexeme/string_spec.rb

@@ -73,6 +73,22 @@ describe Gitlab::Ci::Pipeline::Expression::Lexeme::String do
         expect(token).not_to be_nil
         expect(token.build.evaluate).to eq 'some " string'
       end
+
+      it 'allows to use an empty string inside single quotes' do
+        scanner = StringScanner.new(%(''))
+
+        token = described_class.scan(scanner)
+
+        expect(token.build.evaluate).to eq ''
+      end
+
+      it 'allow to use an empty string inside double quotes' do
+        scanner = StringScanner.new(%(""))
+
+        token = described_class.scan(scanner)
+
+        expect(token.build.evaluate).to eq ''
+      end
     end
   end