Change default value of precision if non is provided in metadata

According to ODATA V4 specification when precision is not provided
it defaults to infinity. ODATA V2 specification does not provide any
information regarding the default value and ODATA V3 specification
is unclear. Thus, following the ODATA V4 specification for all versions
seams as the best idea.

https://www.odata.org/documentation/odata-version-3-0/common-schema-definition-language-csdl/#csdl5.3.4

https://docs.oasis-open.org/odata/odata-csdl-xml/v4.01/csprd06/odata-csdl-xml-v4.01-csprd06.html#sec_Precision

https://docs.oasis-open.org/odata/odata-csdl-xml/v4.01/csprd06/odata-csdl-xml-v4.01-csprd06.html#sec_Scale

Author: mamiksik

CHANGELOG.md

@@ -20,7 +20,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 - Implementation and naming schema of `from_etree` - Martin Miksik
-- Build functions of struct types now handle invalid metadata independently. - Martin Miksik 
+- Build functions of struct types now handle invalid metadata independently. - Martin Miksik
+- Default value of precision if non is provided in metadata - Martin Miksik 
 
 ### Fixed
 - make sure configured error policies are applied for Annotations referencing

pyodata/model/elements.py

@@ -293,7 +293,7 @@ def __init__(self, name, type_info, nullable, max_length, precision, scale):
             self._max_length = int(max_length)
 
         if not precision:
-            self._precision = 0
+            self._precision = None
         else:
             self._precision = int(precision)
         if not scale:
@@ -337,7 +337,7 @@ def scale(self):
         return self._scale
 
     def _check_scale_value(self):
-        if self._scale > self._precision:
+        if self._precision and self._scale > self._precision:
             raise PyODataModelError('Scale value ({}) must be less than or equal to precision value ({})'
                                     .format(self._scale, self._precision))