Return false for empty string

[mattn]

If there are environment variables which have empty value, csvq panic.

csvq interactive shell
Press Ctrl+D or execute "EXIT;" to terminate this shell.

[snip]
panic: runtime error: index out of range

goroutine 1 [running]:
github.com/mithrandie/csvq/lib/cmd.MustBeEnclosed(0xc0001200e0, 0x0, 0xc000086ff0)
	c:/dev/godev/src/github.com/mithrandie/csvq/lib/cmd/utils.go:204 +0x256
github.com/mithrandie/csvq/lib/cmd.EnvironmentVariableSymbol(0xc0001200e0, 0x0, 0x7c4a96, 0x1)
	c:/dev/godev/src/github.com/mithrandie/csvq/lib/cmd/utils.go:191 +0x43
github.com/mithrandie/csvq/lib/query.(*Completer).updateEnvironmentVariables(0xc0000fc000)
	c:/dev/godev/src/github.com/mithrandie/csvq/lib/query/completer_readline.go:268 +0x1bb
github.com/mithrandie/csvq/lib/query.(*Completer).Update(0xc0000fc000)
	c:/dev/godev/src/github.com/mithrandie/csvq/lib/query/completer_readline.go:183 +0x7d
github.com/mithrandie/csvq/lib/query.ReadLineTerminal.UpdateCompleter(...)
	c:/dev/godev/src/github.com/mithrandie/csvq/lib/query/terminal_readline.go:122
github.com/mithrandie/csvq/lib/action.LaunchInteractiveShell(0xc00008fad0, 0x0, 0x0)
	c:/dev/godev/src/github.com/mithrandie/csvq/lib/action/run.go:106 +0x1bd
main.main.func9(0xc0000b6160, 0x0, 0x0)
	c:/dev/godev/src/github.com/mithrandie/csvq/main.go:258 +0x98
github.com/urfave/cli.HandleAction(0x755060, 0xc0000566f0, 0xc0000b6160, 0xc0000b2600, 0x0)
	c:/dev/godev/src/github.com/urfave/cli/app.go:501 +0xcf
github.com/urfave/cli.(*App).Run(0xc0000ce000, 0xc000056170, 0x1, 0x1, 0x0, 0x0)
	c:/dev/godev/src/github.com/urfave/cli/app.go:268 +0x59d
main.main()
	c:/dev/godev/src/github.com/mithrandie/csvq/main.go:276 +0x149a

[codecov]

Codecov Report

Merging #4 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   92.46%   92.46%   +<.01%     
==========================================
  Files          70       70              
  Lines       13408    13410       +2     
==========================================
+ Hits        12398    12400       +2     
  Misses        852      852              
  Partials      158      158

Impacted Files	Coverage Δ
lib/cmd/utils.go	97.12% <100%> (+0.02%)	⬆️

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[mithrandie]

This request is merged and released as version 1.8.2. Thanks.