fix(parser): string concatination with undefined model

[petrovalex]

Related to issue #988

• {{'string' + undefinedModel}} results in 'string'
• {{undefined+undefined}} results in empty string

[vojtajina]

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[petrovalex]

Done. Thanks.

[mhevery]

merged to NEXT

[petrovalex]

@mhevery havn't found in next, am I missing smth?