```ruby
require 'rspec'
require File.join(File.dirname(__FILE__), '..', 'lib/string_calculator')

describe StringCalculator do
  subject { described_class.new }

  it 'returns 0 if empty string provided' do
    expect(subject.add("")).to eq(0)
  end
end
```

Test case for one number provided
user
```ruby
require 'rspec'
require File.join(File.dirname(__FILE__), '..', 'lib/string_calculator')

describe StringCalculator do
  subject { described_class.new }

  it 'returns 0 if empty string provided' do
    expect(subject.add("")).to eq(0)
  end

  it 'returns number if one number provided' do
    expect(subject.add("1")).to eq(1)
  end
end
```
assistant