Consider empty environment variables as set

When assigning values to flags (also when interogatting via
`context.(Global)IsSet`.

For boolean flags, consider empty as `false`.

Using `syscall.Getenv` rather than `os.LookupEnv` in order to support
older Golang versions.

altsrc/flag.go

@@ -2,9 +2,9 @@ package altsrc
 
 import (
 	"fmt"
-	"os"
 	"strconv"
 	"strings"
+	"syscall"
 
 	"gopkg.in/urfave/cli.v1"
 )
@@ -237,13 +237,11 @@ func (f *Float64Flag) ApplyInputSourceValue(context *cli.Context, isc InputSourc
 func isEnvVarSet(envVars string) bool {
 	for _, envVar := range strings.Split(envVars, ",") {
 		envVar = strings.TrimSpace(envVar)
-		if envVal := os.Getenv(envVar); envVal != "" {
+		if _, ok := syscall.Getenv(envVar); ok {
 			// TODO: Can't use this for bools as
 			// set means that it was true or false based on
 			// Bool flag type, should work for other types
-			if len(envVal) > 0 {
-				return true
-			}
+			return true
 		}
 	}