IEV data anomaly (20201217): SOURCE containing empty modifications

Specified with a dash/hyphen but no actual modifications provided.

```
Processing term 802-01-12 (eng)... extract_source_clause 'IEC 62127-1, 3.51, modified - '
[RAW] definition 3.51 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.51",
:relation_type=>{:type=>:modified}}
Processing term 802-01-12 (fra)... extract_source_clause 'IEC 62127-1, 3.51, modified - '
[RAW] définition 3.51 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.51",
:relation_type=>{:type=>:modified}}
Processing term 802-01-13 (eng)... extract_source_clause 'IEC 62127-1, 3.57, modified - '
[RAW] definition 3.57 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.57",
:relation_type=>{:type=>:modified}}
Processing term 802-01-13 (fra)... extract_source_clause 'IEC 62127-1, 3.57, modified - '
[RAW] définition 3.57 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.57",
:relation_type=>{:type=>:modified}}
Processing term 802-01-14 (eng)... extract_source_clause 'IEC 62127-1, 3.65, modified - '
[RAW] definition 3.65 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.65",
:relation_type=>{:type=>:modified}}
Processing term 802-01-14 (fra)... extract_source_clause 'IEC 62127-1, 3.65, modified - '
[RAW] définition 3.65 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.65",
:relation_type=>{:type=>:modified}}
Processing term 802-01-16 (eng)... extract_source_clause 'IEC 62127-1, 3.49, modified - '
[RAW] definition 3.49 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.49",
:relation_type=>{:type=>:modified}}
Processing term 802-01-16 (fra)... extract_source_clause 'IEC 62127-1, 3.49, modified - '
[RAW] définition 3.49 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.49",
:relation_type=>{:type=>:modified}}
Processing term 802-01-17 (eng)... extract_source_clause 'IEC 62127-1, 3.47, modified - '
[RAW] definition 3.47 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.47",
:relation_type=>{:type=>:modified}}
Processing term 802-01-17 (fra)... extract_source_clause 'IEC 62127-1, 3.47, modified - '
[RAW] définition 3.47 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.47",
:relation_type=>{:type=>:modified}}
Processing term 802-01-20 (eng)... extract_source_clause 'IEC 62127-1, 3.11, modified - '
[RAW] definition 3.11 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.11",
:relation_type=>{:type=>:modified}}
Processing term 802-01-20 (fra)... extract_source_clause 'IEC 62127-1, 3.11, modified - '
[RAW] définition 3.11 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.11",
:relation_type=>{:type=>:modified}}
Processing term 802-01-21 (eng)... extract_source_clause 'IEC 62127-1, 3.54, modified - '
[RAW] definition 3.54 of 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.54",
:relation_type=>{:type=>:modified}}
Processing term 802-01-21 (fra)... extract_source_clause 'IEC 62127-1, 3.54, modified - '
[RAW] définition 3.54 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.54",
:relation_type=>{:type=>:modified}}

Processing term 802-01-25 (eng)... extract_source_clause 'IEC 62127-1, 3.44, modified - '
[RAW] definition 3.44 of IEC 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.44",
:relation_type=>{:type=>:modified}}
Processing term 802-01-25 (fra)... extract_source_clause 'IEC 62127-1, 3.44, modified - '
[RAW] définition 3.44 de la CEI 62127-1 MOD
{:source_ref=>"IEC 62127-1",
:clause=>"3.44",
:relation_type=>{:type=>:modified}}
```