Use `ISNULL` in place of `IFNULL` in queries

[anthonysena]

The IFNULL function is not fully supported across all RDBMS platforms and as a result it will fail on those systems that don't support it (i.e. SQL Server). Replacing with ISNULL should fix it. Here are the queries where I found IFNULL:

Characterization/inst/sql/sql_server/RiskFactorBinaryExtraction.sql

Lines 35 to 40 in ca4f13f

	IFNULL(non_cases.covariate_id, cases.covariate_id) as covariate_id,
	IFNULL(non_case_sum_value, 0) as non_case_sum_value,
	IFNULL(case_sum_value, 0) as case_sum_value,
	IFNULL(non_case_average_value, 0) as non_case_average_value,
	IFNULL(case_average_value, 0) as case_average_value,
	(IFNULL(case_average_value, 0.0) - IFNULL(non_case_average_value, 0.0))/

Characterization/inst/sql/sql_server/RiskFactorContinuousExtraction.sql

Lines 37 to 59 in ca4f13f

	IFNULL(non_cases.characterization_case_id, cases.characterization_case_id) as characterization_case_id,
	IFNULL(non_cases.covariate_id, cases.covariate_id) as covariate_id,
	IFNULL(non_case_count_value, 0) as non_case_count_value,
	IFNULL(case_count_value, 0) as case_count_value,
	IFNULL(non_case_min_value, 0) as non_case_min_value,
	IFNULL(case_min_value, 0) as case_min_value,
	IFNULL(non_case_max_value, 0) as non_case_max_value,
	IFNULL(case_max_value, 0) as case_max_value,
	IFNULL(non_case_average_value, 0) as non_case_average_value,
	IFNULL(case_average_value, 0) as case_average_value,
	IFNULL(non_case_median_value, 0) as non_case_median_value,
	IFNULL(case_median_value, 0) as case_median_value,
	IFNULL(non_case_p10_value, 0) as non_case_p10_value,
	IFNULL(case_p10_value, 0) as case_p10_value,
	IFNULL(non_case_p25_value, 0) as non_case_p25_value,
	IFNULL(case_p25_value, 0) as case_p25_value,
	IFNULL(non_case_p75_value, 0) as non_case_p75_value,
	IFNULL(case_p75_value, 0) as case_p75_value,
	IFNULL(non_case_p90_value, 0) as non_case_p90_value,
	IFNULL(case_p90_value, 0) as case_p90_value,
	IFNULL(non_case_standard_deviation, 0) as non_case_standard_deviation,
	IFNULL(case_standard_deviation, 0) as case_standard_deviation,
	(IFNULL(case_average_value, 0.0) - IFNULL(non_case_average_value, 0.0))/