fix(qemu): skip empty QEMU_NET_CIDR instead of erroring (#2568)

os.LookupEnv returns ok=true for an empty string. When QEMU_NET_CIDR
is set to "" (e.g., via a build system that always sets the env var
with a default empty value), the validator rejects it as "empty CIDR"
and the build fails. Skip silently when the value is empty.

Signed-off-by: jmeridth <jmeridth@gmail.com>

Author: jmeridth

pkg/container/qemu_runner.go

@@ -806,7 +806,7 @@ func createMicroVM(ctx context.Context, cfg *Config) error {
 	// The value must be a valid IPv4 CIDR. SLIRP automatically assigns the
 	// gateway, DNS, and DHCP range based on the supplied network.
 	// Example: QEMU_NET_CIDR="192.168.76.0/24"
-	if netCIDR, ok := os.LookupEnv("QEMU_NET_CIDR"); ok {
+	if netCIDR, ok := os.LookupEnv("QEMU_NET_CIDR"); ok && netCIDR != "" {
 		cidr, err := parseAndValidateNetCIDR(netCIDR)
 		if err != nil {
 			return fmt.Errorf("invalid QEMU_NET_CIDR value %q: %w", netCIDR, err)