NoMethodError when invoking `rails` command without arguments (undefined method `start_with?' for nil:NilClass)

[mernen]

With Spring 4.0, running rails without any arguments causes a NoMethodError.

This is caused by b85f41be, since command_name will be nil in that case. I believe a null check is all that's needed.

Steps for reproduction

On a terminal, run the rails command in a Rails project that has Spring 4.0.

Example output

$ rails
Traceback (most recent call last):
    12: from bin/rails:2:in `<main>'
    11: from bin/rails:2:in `load'
    10: from /usr/src/testapp/bin/spring:7:in `<top (required)>'
     9: from /usr/src/testapp/bin/spring:7:in `tap'
     8: from /usr/src/testapp/bin/spring:10:in `block in <top (required)>'
     7: from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:83:in `require'
     6: from /usr/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:83:in `require'
     5: from /usr/local/bundle/gems/spring-4.0.0/lib/spring/binstub.rb:11:in `<top (required)>'
     4: from /usr/local/bundle/gems/spring-4.0.0/lib/spring/binstub.rb:11:in `load'
     3: from /usr/local/bundle/gems/spring-4.0.0/bin/spring:49:in `<top (required)>'
     2: from /usr/local/bundle/gems/spring-4.0.0/lib/spring/client.rb:30:in `run'
     1: from /usr/local/bundle/gems/spring-4.0.0/lib/spring/client/command.rb:7:in `call'
/usr/local/bundle/gems/spring-4.0.0/lib/spring/client/rails.rb:25:in `call': undefined method `start_with?' for nil:NilClass (NoMethodError)