Only display unprintable pattern matches for matches of non-zero length

[egibs]

We currently have an edge case when displaying matches for rules that contain strings which can produce unprintable characters (e.g., rules that are entirely hex strings).

Previously, the file report would show all of the rule's matched string patterns which made it hard to narrow down the actual culprit (falling back to yr scan is usually what I do).

This PR corrects the behavior by only showing pattern matches if the match is of non-zero length.

Before:

$ out/mal --format=strings scan .../usr/bin/kube-proxy
🔎 Scanning ".../usr/bin/kube-proxy"
Matches for .../usr/bin/kube-proxy [CRIT] (2 rules):
ESET_Kobalos [CRIT] (4 strings):
- $encrypted_strings_sizes
- $password_md5_digest
- $rsa_512_mod_header
- $strings_rc4_key
SIGNATURE_BASE_APT_MAL_LNX_Kobalos [CRIT] (4 strings):
- $encrypted_strings_sizes
- $password_md5_digest
- $rsa_512_mod_header
- $strings_rc4_key

💡 For detailed analysis, try "mal analyze <path>"

After:

$ out/mal --format=strings scan ...usr/bin/kube-proxy
🔎 Scanning "...usr/bin/kube-proxy"
Matches for ...usr/bin/kube-proxy [CRIT] (2 rules):
ESET_Kobalos [CRIT] (1 string):
- $rsa_512_mod_header
SIGNATURE_BASE_APT_MAL_LNX_Kobalos [CRIT] (1 string):
- $rsa_512_mod_header

💡 For detailed analysis, try "mal analyze <path>

[stevebeattie]

Woo, nice fix,thanks!