Throws error if replacement input is empty

[gadenbuie]

To reproduce:

• Set text and pattern
• View str_replace so that replacement input is generated
• Leave replacement blank
• Click "Send to console"
• Get error:

Warning: Error in if: missing value where TRUE/FALSE needed
Stack trace (innermost first):
    68: observeEventHandler
     4: shiny::runApp
     3: runGadget
     2: regex_gadget
     1: regexplain:::regexplain_addin
ERROR: [on_request_read] connection reset by peer

Should be quick fix to check that input$replacement is truthy.