argparse: exit with usage if positional arg missing

If a positional (string) arg is missing, the 'Arg()' function will return an
empty string without indicating that the value is missing. Since (for the
moment) positional args are always required, add an explicit check that the
arg is present and, if not, exit with usage.

Signed-off-by: Victoria Dye <vdye@github.com>

Author: vdye

internal/argparse/argparse.go

@@ -168,6 +168,10 @@ func (a *argParser) Parse(ctx context.Context, args []string) {
 			// First, try single string case
 			sPtr, isStr := arg.value.(*string)
 			if isStr {
+				if a.NArg() == 0 {
+					a.Usage(ctx, "No value specified for required argument '%s'", arg.name)
+				}
+
 				*sPtr = a.Arg(0)
 				a.argOffset++
 				continue