Use empty posting while balancing imported entry

[dumbPy]

If an empty posting i.e., one without an amount, is entered by hand,
it should be used to balance the transaction instead of a FIXME posting.

Example

2020-08-26 * "Loan to Asim"
    Assets:INR:HSBC:Saving -34000 INR
    Assets:Receivables:Asim

when this transaction is imported currently, it gets balanced as

2020-08-26 * "Loan to Asim"
    Assets:INR:HSBC:Saving -34000 INR
    Assets:Receivables:Asim 
    Assets:FIXME            34000 INR

but should be, after this PR

2020-08-26 * "Loan to Asim"
    Assets:INR:HSBC:Saving -34000 INR
    Assets:Receivables:Asim 34000 INR

[coveralls]

Pull Request Test Coverage Report for Build 213

• 8 of 13 (61.54%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 68.862%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
beancount_import/source/generic_importer_source.py	8	13	61.54%

Totals
Change from base Build 210:	-0.02%
Covered Lines:	5150
Relevant Lines:	7243

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💛 - Coveralls

[jbms]

Thanks, this looks good, but can you please add a unit test for this?