Fix null dereference when passing empty slice to Base64.encode (#12377)

spec/std/base64_spec.cr

@@ -48,6 +48,12 @@ describe "Base64" do
     assert_prints base64_strict_encode(slice), "AQEBAQE="
   end
 
+  it "encodes empty slice" do
+    slice = Bytes.empty
+    assert_prints base64_encode(slice), ""
+    assert_prints base64_strict_encode(slice), ""
+  end
+
   it "encodes static array" do
     array = uninitialized StaticArray(UInt8, 5)
     (0...5).each { |i| array[i] = 1_u8 }

src/base64.cr

@@ -203,6 +203,7 @@ module Base64
     bytes = chars.to_unsafe
     size = data.size
     cstr = data.to_unsafe
+    return if cstr.null? || size == 0
     endcstr = cstr + size - size % 3 - 3
 
     # process bunch of full triples