The file does not seem to be empty. This is what is in the file:
$ hexdump server_key.der
0000000 8230 2201 0d30 0906 862a 8648 0df7 0101
0000010 0501 0300 0182 000f 8230 0a01 8202 0101
0000020 be00 becc db43 ea8e 2715 bba6 6d52 51e1
0000030 a02b ccab 64a1 4877 7cad fc66 7f80 99f6
0000040 25a5 f2f2 e0da cf43 263a 9ba4 87a1 0e03
0000050 8d9a 9a23 eabc ea99 d368 145a 26b1 bd0f
0000060 6daa 0c6f c4ac 2c77 c5d1 b1c8 7bd1 e068
0000070 7325 527b 6889 bd20 c606 e6f0 3000 e0c0
0000080 f6cf 3a1b e945 5bc4 1755 a306 4ad3 d5c6
0000090 d2b8 0217 27b5 8d7d d4e4 d37d c0ed 8a1d
00000a0 257c 211e 514a 57ae dd06 bc60 34a1 aa90
00000b0 09cc 3b9e 413a 3c4c f39d fdfd 27b7 59c1
00000c0 9881 6054 624a a47a bf9a 92df 3e1b a7fc
00000d0 a4e4 3ab3 5f9a 9357 eb8e 6419 2295 2c4a
00000e0 60d5 f9f5 03d0 8350 c069 536b f0f0 f8da
00000f0 8213 cc1f 5fbb c1e2 3adf 7fe9 e25d b97d
0000100 8050 583c ef33 f38c 3f80 0111 68d2 5f86
0000110 5e3c c1e6 328e 282b b5cb 1bcc 50a8 a75e
0000120 020d 0103 0100 ffff ffff ffff ffff ffff
0000130 ffff ffff ffff ffff ffff ffff ffff ffff
(Attachment server_key.txt is missing)