Fix number formatting when cells are empty

[ldodds]

If a spreadsheet has empty cells formatted as numbers then attempting to turn the worksheet into CSV fails with an exception:

	27: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/formatters/csv.rb:12:in `to_csv'
	26: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/formatters/csv.rb:24:in `write_csv_content'
	25: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx.rb:134:in `first_row'
	24: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet.rb:65:in `first_row'
	23: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet.rb:126:in `first_last_row_col'
	22: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet.rb:22:in `cells'
	21: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:20:in `cells'
	20: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:214:in `extract_cells'
	19: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:214:in `with_index'
	18: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:234:in `each'
	17: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:234:in `upto'
	16: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:235:in `block in each'
	15: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:215:in `block in extract_cells'
	14: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:215:in `with_index'
	13: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:234:in `each'
	12: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:234:in `upto'
	11: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:235:in `block in each'
	10: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:224:in `block (2 levels) in extract_cells'
	 9: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:101:in `cell_from_xml'
	 8: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:234:in `each'
	 7: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:234:in `upto'
	 6: from /home/ldodds/.rvm/gems/ruby-2.7.6/gems/nokogiri-1.15.4-x86_64-linux/lib/nokogiri/xml/node_set.rb:235:in `block in each'
	 5: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:114:in `block in cell_from_xml'
	 4: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:172:in `create_cell_from_value'
	 3: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/sheet_doc.rb:172:in `new'
	 2: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/cell/number.rb:16:in `initialize'
	 1: from /home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/cell/number.rb:26:in `create_numeric'
/home/ldodds/.rvm/gems/ruby-2.7.6/bundler/gems/roo-7732ee32d43b/lib/roo/excelx/cell/number.rb:26:in `Float': invalid value for Float(): "" (ArgumentError)

The Number class does not check whether the provided cell value is empty before attempting to parse it into a Float or format it back into a String.

In our example, the given cells in the spreadsheet had an empty String as their value, and an excelx_value of [:numeric_or_formula, "#,##0.00"].

This PR

• adds a minimal test for the Number class, for its constructor and formatted_value methods, which will demonstrate the bug
• a fix to create_numeric to return nil if the value is empty or nil
• a fix to formatted_value to return "" if the cell_value is empty or nil