fix(runStaticServer): Don't fail when trying to open browser

[gadenbuie]

Encountered in #393

Before

library(httpuv)
options(browser = character())

runStaticServer(
  system.file("example-static-site", package = "httpuv"),
  browse = TRUE
)
#> Serving: '/Users/garrick/Library/R/arm64/4.3/library/httpuv/example-static-site'
#> View at: http://127.0.0.1:7446
#> Error in utils::browseURL(paste0("http://", host, ":", port)): 'browser' must be a non-empty character string

After

library(httpuv)
options(browser = character())

s <- runStaticServer(
  system.file("example-static-site", package = "httpuv"),
  browse = TRUE,
  background = TRUE
)
#> Serving: '/Users/garrick/work/rstudio/httpuv/inst/example-static-site'
#> View at: http://127.0.0.1:34950
#> Could not open browser due to error in `utils::browseURL()`: 'browser' must be a non-empty character string
s$stop()